repart: Add tests for supplement partitions

This was designed to deal with $BOOT, as defined by the Boot Loader
Specification, but it was made a generic mechanism because it is useful
elsewhere too. See the updated man page for usage examples, motivation,
and an explanation of how this works.

man/repart.d.xml

@@ -76,16 +76,7 @@
         <term><varname>Type=</varname></term>
 
         <listitem><para>The GPT partition type UUID to match. This may be a GPT partition type UUID such as
-        <constant>4f68bce3-e8cd-4db1-96e7-fbcaf984b709</constant>, or an identifier.
-        Architecture specific partition types can use one of these architecture identifiers:
-        <constant>alpha</constant>, <constant>arc</constant>, <constant>arm</constant> (32-bit),
-        <constant>arm64</constant> (64-bit, aka aarch64), <constant>ia64</constant>,
-        <constant>loongarch64</constant>, <constant>mips-le</constant>, <constant>mips64-le</constant>,
-        <constant>parisc</constant>, <constant>ppc</constant>, <constant>ppc64</constant>,
-        <constant>ppc64-le</constant>, <constant>riscv32</constant>, <constant>riscv64</constant>,
-        <constant>s390</constant>, <constant>s390x</constant>, <constant>tilegx</constant>,
-        <constant>x86</constant> (32-bit, aka i386) and <constant>x86-64</constant> (64-bit, aka amd64).
-        </para>
+        <constant>4f68bce3-e8cd-4db1-96e7-fbcaf984b709</constant>, or an identifier.</para>
 
         <para>The supported identifiers are:</para>
 
@@ -237,7 +228,14 @@
           </tgroup>
         </table>
 
-        <para>This setting defaults to <constant>linux-generic</constant>.</para>
+        <para>Architecture specific partition types can use one of these architecture identifiers:
+        <constant>alpha</constant>, <constant>arc</constant>, <constant>arm</constant> (32-bit),
+        <constant>arm64</constant> (64-bit, aka aarch64), <constant>ia64</constant>,
+        <constant>loongarch64</constant>, <constant>mips-le</constant>, <constant>mips64-le</constant>,
+        <constant>parisc</constant>, <constant>ppc</constant>, <constant>ppc64</constant>,
+        <constant>ppc64-le</constant>, <constant>riscv32</constant>, <constant>riscv64</constant>,
+        <constant>s390</constant>, <constant>s390x</constant>, <constant>tilegx</constant>,
+        <constant>x86</constant> (32-bit, aka i386) and <constant>x86-64</constant> (64-bit, aka amd64).</para>
 
         <para>Most of the partition type UUIDs listed above are defined in the <ulink
         url="https://uapi-group.org/specifications/specs/discoverable_partitions_specification">Discoverable Partitions
@@ -897,6 +895,59 @@
 
         <xi:include href="version-info.xml" xpointer="v257"/></listitem>
       </varlistentry>
+
+      <varlistentry>
+        <term><varname>SupplementFor=</varname></term>
+
+        <listitem><para>Takes a partition definition name, such as <literal>10-esp</literal>. If specified,
+        <command>systemd-repart</command> will avoid creating this partition and instead prefer to partially
+        merge the two definitions. However, depending on the existing layout of partitions on disk,
+        <command>systemd-repart</command> may be forced to fall back onto un-merging the definitions and
+        using them as originally written, potentially creating this partition. Specifically,
+        <command>systemd-repart</command> will fall back if this partition is found to already exist on disk,
+        or if the target partition already exists on disk but is too small, or if it cannot allocate space
+        for the merged partition for some other reason.</para>
+
+        <para>The following fields are merged into the target definition in the specified ways:
+        <varname>Weight=</varname> and <varname>PaddingWeight=</varname> are simply overwritten;
+        <varname>SizeMinBytes=</varname> and <varname>PaddingMinBytes=</varname> use the larger of the two
+        values; <varname>SizeMaxBytes=</varname> and <varname>PaddingMaxBytes=</varname> use the smaller
+        value; and <varname>CopyFiles=</varname>, <varname>ExcludeFiles=</varname>,
+        <varname>ExcludeFilesTarget=</varname>, <varname>MakeDirectories=</varname>, and
+        <varname>Subvolumes=</varname> are concatenated.</para>
+
+        <para>Usage of this option in combination with <varname>CopyBlocks=</varname>,
+        <varname>Encrypt=</varname>, or <varname>Verity=</varname> is not supported. The target definition
+        cannot set these settings either. A definition cannot simultaneously be a supplement and act as a
+        target for some other supplement definition. A target cannot have more than one supplement partition
+        associated with it.</para>
+
+        <para>For example, distributions can use this to implement <variable>$BOOT</variable> as defined in
+        the <ulink url="https://uapi-group.org/specifications/specs/boot_loader_specification/">Boot Loader
+        Specification</ulink>. Distributions may prefer to use the ESP as <variable>$BOOT</variable> whenever
+        possible, but to adhere to the spec XBOOTLDR must sometimes be used instead. So, they should create
+        two definitions: the first defining an ESP big enough to hold just the bootloader, and a second for
+        the XBOOTLDR that's sufficiently large to hold kernels and configured as a supplement for the ESP.
+        Whenever possible, <command>systemd-repart</command> will try to merge the two definitions to create
+        one large ESP, but if that's not allowable due to the existing conditions on disk a small ESP and a
+        large XBOOTLDR will be created instead.</para>
+
+        <para>As another example, distributions can also use this to seamlessly share a single
+        <filename>/home</filename> partition in a multi-boot scenario, while preferring to keep
+        <filename>/home</filename> on the root partition by default. Having a <filename>/home</filename>
+        partition separated from the root partition entails some extra complexity: someone has to decide how
+        to split the space between the two partitions. On the other hand, it allows a user to share their
+        home area between multiple installed OSs (i.e. via <citerefentry><refentrytitle>systemd-homed.service
+        </refentrytitle><manvolnum>8</manvolnum></citerefentry>). Distributions should create two definitions:
+        the first for a root partition that takes up some relatively small percentage of the disk, and the
+        second as a supplement for the first to create a <filename>/home</filename> partition that takes up
+        all the remaining free space. On first boot, if <command>systemd-repart</command> finds an existing
+        <filename>/home</filename> partition on disk, it'll un-merge the definitions and create just a small
+        root partition. Otherwise, the definitions will be merged and a single large root partition will be
+        created.</para>
+
+        <xi:include href="version-info.xml" xpointer="v257"/></listitem>
+      </varlistentry>
     </variablelist>
   </refsect1>
 

src/basic/strv.h

@@ -153,7 +153,7 @@ bool strv_overlap(char * const *a, char * const *b) _pure_;
         _STRV_FOREACH_BACKWARDS(s, l, UNIQ_T(h, UNIQ), UNIQ_T(i, UNIQ))
 
 #define _STRV_FOREACH_PAIR(x, y, l, i)                          \
-        for (typeof(*l) *x, *y, *i = (l);                       \
+        for (typeof(*(l)) *x, *y, *i = (l);                     \
              i && *(x = i) && *(y = i + 1);                     \
              i += 2)
 

src/partition/repart.c

@@ -404,6 +404,10 @@ typedef struct Partition {
 
         PartitionEncryptedVolume *encrypted_volume;
 
+        char *supplement_for_name;
+        struct Partition *supplement_for, *supplement_target_for;
+        struct Partition *suppressing;
+
         struct Partition *siblings[_VERITY_MODE_MAX];
 
         LIST_FIELDS(struct Partition, partitions);
@@ -411,6 +415,7 @@ typedef struct Partition {
 
 #define PARTITION_IS_FOREIGN(p) (!(p)->definition_path)
 #define PARTITION_EXISTS(p) (!!(p)->current_partition)
+#define PARTITION_SUPPRESSED(p) ((p)->supplement_for && (p)->supplement_for->suppressing == (p))
 
 struct FreeArea {
         Partition *after;
@@ -520,6 +525,28 @@ static Partition *partition_new(void) {
         return p;
 }
 
+static void partition_unlink_supplement(Partition *p) {
+        assert(p);
+
+        assert(!p->supplement_for || !p->supplement_target_for); /* Can't be both */
+
+        if (p->supplement_target_for) {
+                assert(p->supplement_target_for->supplement_for == p);
+
+                p->supplement_target_for->supplement_for = NULL;
+        }
+
+        if (p->supplement_for) {
+                assert(p->supplement_for->supplement_target_for == p);
+                assert(!p->supplement_for->suppressing || p->supplement_for->suppressing == p);
+
+                p->supplement_for->supplement_target_for = p->supplement_for->suppressing = NULL;
+        }
+
+        p->supplement_for_name = mfree(p->supplement_for_name);
+        p->supplement_target_for = p->supplement_for = p->suppressing = NULL;
+}
+
 static Partition* partition_free(Partition *p) {
         if (!p)
                 return NULL;
@@ -563,6 +590,8 @@ static Partition* partition_free(Partition *p) {
 
         partition_encrypted_volume_free(p->encrypted_volume);
 
+        partition_unlink_supplement(p);
+
         return mfree(p);
 }
 
@@ -608,6 +637,8 @@ static void partition_foreignize(Partition *p) {
         p->n_mountpoints = 0;
 
         p->encrypted_volume = partition_encrypted_volume_free(p->encrypted_volume);
+
+        partition_unlink_supplement(p);
 }
 
 static bool partition_type_exclude(const GptPartitionType *type) {
@@ -740,6 +771,10 @@ static void partition_drop_or_foreignize(Partition *p) {
 
                 p->dropped = true;
                 p->allocated_to_area = NULL;
+
+                /* If a supplement partition is dropped, we don't want to merge in its settings. */
+                if (PARTITION_SUPPRESSED(p))
+                        p->supplement_for->suppressing = NULL;
         }
 }
 
@@ -775,7 +810,7 @@ static bool context_drop_or_foreignize_one_priority(Context *context) {
 }
 
 static uint64_t partition_min_size(const Context *context, const Partition *p) {
-        uint64_t sz;
+        uint64_t sz, override_min;
 
         assert(context);
         assert(p);
@@ -817,11 +852,13 @@ static uint64_t partition_min_size(const Context *context, const Partition *p) {
                         sz = d;
         }
 
-        return MAX(round_up_size(p->size_min != UINT64_MAX ? p->size_min : DEFAULT_MIN_SIZE, context->grain_size), sz);
+        override_min = p->suppressing ? MAX(p->size_min, p->suppressing->size_min) : p->size_min;
+
+        return MAX(round_up_size(override_min != UINT64_MAX ? override_min : DEFAULT_MIN_SIZE, context->grain_size), sz);
 }
 
 static uint64_t partition_max_size(const Context *context, const Partition *p) {
-        uint64_t sm;
+        uint64_t sm, override_max;
 
         /* Calculate how large the partition may become at max. This is generally the configured maximum
          * size, except when it already exists and is larger than that. In that case it's the existing size,
@@ -839,10 +876,11 @@ static uint64_t partition_max_size(const Context *context, const Partition *p) {
         if (p->verity == VERITY_SIG)
                 return VERITY_SIG_SIZE;
 
-        if (p->size_max == UINT64_MAX)
+        override_max = p->suppressing ? MIN(p->size_max, p->suppressing->size_max) : p->size_max;
+        if (override_max == UINT64_MAX)
                 return UINT64_MAX;
 
-        sm = round_down_size(p->size_max, context->grain_size);
+        sm = round_down_size(override_max, context->grain_size);
 
         if (p->current_size != UINT64_MAX)
                 sm = MAX(p->current_size, sm);
@@ -851,13 +889,17 @@ static uint64_t partition_max_size(const Context *context, const Partition *p) {
 }
 
 static uint64_t partition_min_padding(const Partition *p) {
+        uint64_t override_min;
+
         assert(p);
-        return p->padding_min != UINT64_MAX ? p->padding_min : 0;
+
+        override_min = p->suppressing ? MAX(p->padding_min, p->suppressing->padding_min) : p->padding_min;
+        return override_min != UINT64_MAX ? override_min : 0;
 }
 
 static uint64_t partition_max_padding(const Partition *p) {
         assert(p);
-        return p->padding_max;
+        return p->suppressing ? MIN(p->padding_max, p->suppressing->padding_max) : p->padding_max;
 }
 
 static uint64_t partition_min_size_with_padding(Context *context, const Partition *p) {
@@ -977,14 +1019,22 @@ static bool context_allocate_partitions(Context *context, uint64_t *ret_largest_
                 uint64_t required;
                 FreeArea *a = NULL;
 
-                /* Skip partitions we already dropped or that already exist */
-                if (p->dropped || PARTITION_EXISTS(p))
+                if (p->dropped || PARTITION_IS_FOREIGN(p) || PARTITION_SUPPRESSED(p))
                         continue;
 
                 /* How much do we need to fit? */
                 required = partition_min_size_with_padding(context, p);
                 assert(required % context->grain_size == 0);
 
+                /* For existing partitions, we should verify that they'll actually fit */
+                if (PARTITION_EXISTS(p)) {
+                        if (p->current_size + p->current_padding < required)
+                                return false; /* 😢 We won't be able to grow to the required min size! */
+
+                        continue;
+                }
+
+                /* For new partitions, see if there's a free area big enough */
                 for (size_t i = 0; i < context->n_free_areas; i++) {
                         a = context->free_areas[i];
 
@@ -1007,6 +1057,57 @@ static bool context_allocate_partitions(Context *context, uint64_t *ret_largest_
         return true;
 }
 
+static bool context_unmerge_and_allocate_partitions(Context *context) {
+        assert(context);
+
+        /* This should only be called after plain context_allocate_partitions fails. This algorithm will
+         * try, in the order that minimizes the number of created supplement partitions, all combinations of
+         * un-suppressing supplement partitions until it finds one that works. */
+
+        /* First, let's try to un-suppress just one supplement partition and see if that gets us anywhere */
+        LIST_FOREACH(partitions, p, context->partitions) {
+                Partition *unsuppressed;
+
+                if (!p->suppressing)
+                        continue;
+
+                unsuppressed = TAKE_PTR(p->suppressing);
+
+                if (context_allocate_partitions(context, NULL))
+                        return true;
+
+                p->suppressing = unsuppressed;
+        }
+
+        /* Looks like not. So we have to un-suppress at least two partitions. We can do this recursively */
+        LIST_FOREACH(partitions, p, context->partitions) {
+                Partition *unsuppressed;
+
+                if (!p->suppressing)
+                        continue;
+
+                unsuppressed = TAKE_PTR(p->suppressing);
+
+                if (context_unmerge_and_allocate_partitions(context))
+                        return true;
+
+                p->suppressing = unsuppressed;
+        }
+
+        /* No combination of un-suppressed supplements made it possible to fit the partitions */
+        return false;
+}
+
+static uint32_t partition_weight(const Partition *p) {
+        assert(p);
+        return p->suppressing ? p->suppressing->weight : p->weight;
+}
+
+static uint32_t partition_padding_weight(const Partition *p) {
+        assert(p);
+        return p->suppressing ? p->suppressing->padding_weight : p->padding_weight;
+}
+
 static int context_sum_weights(Context *context, FreeArea *a, uint64_t *ret) {
         uint64_t weight_sum = 0;
 
@@ -1020,13 +1121,11 @@ static int context_sum_weights(Context *context, FreeArea *a, uint64_t *ret) {
                 if (p->padding_area != a && p->allocated_to_area != a)
                         continue;
 
-                if (p->weight > UINT64_MAX - weight_sum)
+                if (!INC_SAFE(&weight_sum, partition_weight(p)))
                         goto overflow_sum;
-                weight_sum += p->weight;
 
-                if (p->padding_weight > UINT64_MAX - weight_sum)
+                if (!INC_SAFE(&weight_sum, partition_padding_weight(p)))
                         goto overflow_sum;
-                weight_sum += p->padding_weight;
         }
 
         *ret = weight_sum;
@@ -1091,7 +1190,6 @@ static bool context_grow_partitions_phase(
          * get any additional room from the left-overs. Similar, if two partitions have the same weight they
          * should get the same space if possible, even if one has a smaller minimum size than the other. */
         LIST_FOREACH(partitions, p, context->partitions) {
-
                 /* Look only at partitions associated with this free area, i.e. immediately
                  * preceding it, or allocated into it */
                 if (p->allocated_to_area != a && p->padding_area != a)
@@ -1099,11 +1197,14 @@ static bool context_grow_partitions_phase(
 
                 if (p->new_size == UINT64_MAX) {
                         uint64_t share, rsz, xsz;
+                        uint32_t weight;
                         bool charge = false;
 
+                        weight = partition_weight(p);
+
                         /* Calculate how much this space this partition needs if everyone would get
                          * the weight based share */
-                        share = scale_by_weight(*span, p->weight, *weight_sum);
+                        share = scale_by_weight(*span, weight, *weight_sum);
 
                         rsz = partition_min_size(context, p);
                         xsz = partition_max_size(context, p);
@@ -1143,15 +1244,18 @@ static bool context_grow_partitions_phase(
 
                         if (charge) {
                                 *span = charge_size(context, *span, p->new_size);
-                                *weight_sum = charge_weight(*weight_sum, p->weight);
+                                *weight_sum = charge_weight(*weight_sum, weight);
                         }
                 }
 
                 if (p->new_padding == UINT64_MAX) {
                         uint64_t share, rsz, xsz;
+                        uint32_t padding_weight;
                         bool charge = false;
 
-                        share = scale_by_weight(*span, p->padding_weight, *weight_sum);
+                        padding_weight = partition_padding_weight(p);
+
+                        share = scale_by_weight(*span, padding_weight, *weight_sum);
 
                         rsz = partition_min_padding(p);
                         xsz = partition_max_padding(p);
@@ -1170,7 +1274,7 @@ static bool context_grow_partitions_phase(
 
                         if (charge) {
                                 *span = charge_size(context, *span, p->new_padding);
-                                *weight_sum = charge_weight(*weight_sum, p->padding_weight);
+                                *weight_sum = charge_weight(*weight_sum, padding_weight);
                         }
                 }
         }
@@ -2155,7 +2259,9 @@ static int partition_finalize_fstype(Partition *p, const char *path) {
 
 static bool partition_needs_populate(const Partition *p) {
         assert(p);
-        return !strv_isempty(p->copy_files) || !strv_isempty(p->make_directories) || !strv_isempty(p->make_symlinks);
+        assert(!p->supplement_for || !p->suppressing); /* Avoid infinite recursion */
+        return !strv_isempty(p->copy_files) || !strv_isempty(p->make_directories) || !strv_isempty(p->make_symlinks) ||
+                (p->suppressing && partition_needs_populate(p->suppressing));
 }
 
 static int partition_read_definition(Partition *p, const char *path, const char *const *conf_file_dirs) {
@@ -2196,6 +2302,7 @@ static int partition_read_definition(Partition *p, const char *path, const char
                 { "Partition", "EncryptedVolume",          config_parse_encrypted_volume,  0,                                  p                           },
                 { "Partition", "Compression",              config_parse_string,            CONFIG_PARSE_STRING_SAFE_AND_ASCII, &p->compression             },
                 { "Partition", "CompressionLevel",         config_parse_string,            CONFIG_PARSE_STRING_SAFE_AND_ASCII, &p->compression_level       },
+                { "Partition", "SupplementFor",            config_parse_string,            0,                                  &p->supplement_for_name     },
                 {}
         };
         _cleanup_free_ char *filename = NULL;
@@ -2320,6 +2427,18 @@ static int partition_read_definition(Partition *p, const char *path, const char
                 return log_syntax(NULL, LOG_ERR, path, 1, SYNTHETIC_ERRNO(EINVAL),
                                   "DefaultSubvolume= must be one of the paths in Subvolumes=.");
 
+        if (p->supplement_for_name) {
+                if (!filename_part_is_valid(p->supplement_for_name))
+                        return log_syntax(NULL, LOG_ERR, path, 1, SYNTHETIC_ERRNO(EINVAL),
+                                          "SupplementFor= is an invalid filename: %s",
+                                          p->supplement_for_name);
+
+                if (p->copy_blocks_path || p->copy_blocks_auto || p->encrypt != ENCRYPT_OFF ||
+                    p->verity != VERITY_OFF)
+                        return log_syntax(NULL, LOG_ERR, path, 1, SYNTHETIC_ERRNO(EINVAL),
+                                          "SupplementFor= cannot be combined with CopyBlocks=/Encrypt=/Verity=");
+        }
+
         /* Verity partitions are read only, let's imply the RO flag hence, unless explicitly configured otherwise. */
         if ((IN_SET(p->type.designator,
                     PARTITION_ROOT_VERITY,
@@ -2626,6 +2745,58 @@ static int context_copy_from(Context *context) {
         return 0;
 }
 
+static bool check_cross_def_ranges_valid(uint64_t a_min, uint64_t a_max, uint64_t b_min, uint64_t b_max) {
+        if (a_min == UINT64_MAX && b_min == UINT64_MAX)
+                return true;
+
+        if (a_max == UINT64_MAX && b_max == UINT64_MAX)
+                return true;
+
+        return MAX(a_min != UINT64_MAX ? a_min : 0, b_min != UINT64_MAX ? b_min : 0) <= MIN(a_max, b_max);
+}
+
+static int supplement_find_target(const Context *context, const Partition *supplement, Partition **ret) {
+        int r;
+
+        assert(context);
+        assert(supplement);
+        assert(ret);
+
+        LIST_FOREACH(partitions, p, context->partitions) {
+                _cleanup_free_ char *filename = NULL;
+
+                if (p == supplement)
+                        continue;
+
+                r = path_extract_filename(p->definition_path, &filename);
+                if (r < 0)
+                        return log_error_errno(r,
+                                               "Failed to extract filename from path '%s': %m",
+                                               p->definition_path);
+
+                *ASSERT_PTR(endswith(filename, ".conf")) = 0; /* Remove the file extension */
+
+                if (!streq(supplement->supplement_for_name, filename))
+                        continue;
+
+                if (p->supplement_for_name)
+                        return log_syntax(NULL, LOG_ERR, supplement->definition_path, 1, SYNTHETIC_ERRNO(EINVAL),
+                                          "SupplementFor= target is itself configured as a supplement.");
+
+                if (p->suppressing)
+                        return log_syntax(NULL, LOG_ERR, supplement->definition_path, 1, SYNTHETIC_ERRNO(EINVAL),
+                                          "SupplementFor= target already has a supplement defined: %s",
+                                          p->suppressing->definition_path);
+
+                *ret = p;
+                return 0;
+        }
+
+        return log_syntax(NULL, LOG_ERR, supplement->definition_path, 1, SYNTHETIC_ERRNO(EINVAL),
+                          "Couldn't find target partition for SupplementFor=%s",
+                          supplement->supplement_for_name);
+}
+
 static int context_read_definitions(Context *context) {
         _cleanup_strv_free_ char **files = NULL;
         Partition *last = LIST_FIND_TAIL(partitions, context->partitions);
@@ -2717,7 +2888,33 @@ static int context_read_definitions(Context *context) {
                 if (dp->minimize == MINIMIZE_OFF && !(dp->copy_blocks_path || dp->copy_blocks_auto))
                         return log_syntax(NULL, LOG_ERR, p->definition_path, 1, SYNTHETIC_ERRNO(EINVAL),
                                           "Minimize= set for verity hash partition but data partition does not set CopyBlocks= or Minimize=.");
+        }
 
+        LIST_FOREACH(partitions, p, context->partitions) {
+                Partition *tgt = NULL;
+
+                if (!p->supplement_for_name)
+                        continue;
+
+                r = supplement_find_target(context, p, &tgt);
+                if (r < 0)
+                        return r;
+
+                if (tgt->copy_blocks_path || tgt->copy_blocks_auto || tgt->encrypt != ENCRYPT_OFF ||
+                    tgt->verity != VERITY_OFF)
+                        return log_syntax(NULL, LOG_ERR, p->definition_path, 1, SYNTHETIC_ERRNO(EINVAL),
+                                          "SupplementFor= target uses CopyBlocks=/Encrypt=/Verity=");
+
+                if (!check_cross_def_ranges_valid(p->size_min, p->size_max, tgt->size_min, tgt->size_max))
+                        return log_syntax(NULL, LOG_ERR, p->definition_path, 1, SYNTHETIC_ERRNO(EINVAL),
+                                          "SizeMinBytes= larger than SizeMaxBytes= when merged with SupplementFor= target.");
+
+                if (!check_cross_def_ranges_valid(p->padding_min, p->padding_max, tgt->padding_min, tgt->padding_max))
+                        return log_syntax(NULL, LOG_ERR, p->definition_path, 1, SYNTHETIC_ERRNO(EINVAL),
+                                          "PaddingMinBytes= larger than PaddingMaxBytes= when merged with SupplementFor= target.");
+
+                p->supplement_for = tgt;
+                tgt->suppressing = tgt->supplement_target_for = p;
         }
 
         return 0;
@@ -3101,6 +3298,10 @@ static int context_load_partition_table(Context *context) {
                 }
         }
 
+        LIST_FOREACH(partitions, p, context->partitions)
+                if (PARTITION_SUPPRESSED(p) && PARTITION_EXISTS(p))
+                        p->supplement_for->suppressing = NULL;
+
 add_initial_free_area:
         nsectors = fdisk_get_nsectors(c);
         assert(nsectors <= UINT64_MAX/secsz);
@@ -3192,6 +3393,11 @@ static void context_unload_partition_table(Context *context) {
 
                 p->current_uuid = SD_ID128_NULL;
                 p->current_label = mfree(p->current_label);
+
+                /* A supplement partition is only ever un-suppressed if the existing partition table prevented
+                 * us from suppressing it. So when unloading the partition table, we must re-suppress. */
+                if (p->supplement_for)
+                        p->supplement_for->suppressing = p;
         }
 
         context->start = UINT64_MAX;
@@ -4969,6 +5175,31 @@ static int add_exclude_path(const char *path, Hashmap **denylist, DenyType type)
         return 0;
 }
 
+static int shallow_join_strv(char ***ret, char **a, char **b) {
+        _cleanup_free_ char **joined = NULL;
+        char **iter;
+
+        assert(ret);
+
+        joined = new(char*, strv_length(a) + strv_length(b) + 1);
+        if (!joined)
+                return log_oom();
+
+        iter = joined;
+
+        STRV_FOREACH(i, a)
+                *(iter++) = *i;
+
+        STRV_FOREACH(i, b)
+                if (!strv_contains(joined, *i))
+                        *(iter++) = *i;
+
+        *iter = NULL;
+
+        *ret = TAKE_PTR(joined);
+        return 0;
+}
+
 static int make_copy_files_denylist(
                 Context *context,
                 const Partition *p,
@@ -4977,6 +5208,7 @@ static int make_copy_files_denylist(
                 Hashmap **ret) {
 
         _cleanup_hashmap_free_ Hashmap *denylist = NULL;
+        _cleanup_free_ char **override_exclude_src = NULL, **override_exclude_tgt = NULL;
         int r;
 
         assert(context);
@@ -4996,13 +5228,26 @@ static int make_copy_files_denylist(
 
         /* Add the user configured excludes. */
 
-        STRV_FOREACH(e, p->exclude_files_source) {
+        if (p->suppressing) {
+                r = shallow_join_strv(&override_exclude_src,
+                                      p->exclude_files_source,
+                                      p->suppressing->exclude_files_source);
+                if (r < 0)
+                        return r;
+                r = shallow_join_strv(&override_exclude_tgt,
+                                      p->exclude_files_target,
+                                      p->suppressing->exclude_files_target);
+                if (r < 0)
+                        return r;
+        }
+
+        STRV_FOREACH(e, override_exclude_src ?: p->exclude_files_source) {
                 r = add_exclude_path(*e, &denylist, endswith(*e, "/") ? DENY_CONTENTS : DENY_INODE);
                 if (r < 0)
                         return r;
         }
 
-        STRV_FOREACH(e, p->exclude_files_target) {
+        STRV_FOREACH(e, override_exclude_tgt ?: p->exclude_files_target) {
                 _cleanup_free_ char *path = NULL;
 
                 const char *s = path_startswith(*e, target);
@@ -5096,6 +5341,7 @@ static int add_subvolume_path(const char *path, Set **subvolumes) {
 static int make_subvolumes_strv(const Partition *p, char ***ret) {
         _cleanup_strv_free_ char **subvolumes = NULL;
         Subvolume *subvolume;
+        int r;
 
         assert(p);
         assert(ret);
@@ -5104,6 +5350,18 @@ static int make_subvolumes_strv(const Partition *p, char ***ret) {
                 if (strv_extend(&subvolumes, subvolume->path) < 0)
                         return log_oom();
 
+        if (p->suppressing) {
+                _cleanup_strv_free_ char **suppressing = NULL;
+
+                r = make_subvolumes_strv(p->suppressing, &suppressing);
+                if (r < 0)
+                        return r;
+
+                r = strv_extend_strv(&subvolumes, suppressing, /* filter_duplicates= */ true);
+                if (r < 0)
+                        return log_oom();
+        }
+
         *ret = TAKE_PTR(subvolumes);
         return 0;
 }
@@ -5114,18 +5372,22 @@ static int make_subvolumes_set(
                 const char *target,
                 Set **ret) {
 
+        _cleanup_strv_free_ char **paths = NULL;
         _cleanup_set_free_ Set *subvolumes = NULL;
-        Subvolume *subvolume;
         int r;
 
         assert(p);
         assert(target);
         assert(ret);
 
-        ORDERED_HASHMAP_FOREACH(subvolume, p->subvolumes) {
+        r = make_subvolumes_strv(p, &paths);
+        if (r < 0)
+                return r;
+
+        STRV_FOREACH(subvolume, paths) {
                 _cleanup_free_ char *path = NULL;
 
-                const char *s = path_startswith(subvolume->path, target);
+                const char *s = path_startswith(*subvolume, target);
                 if (!s)
                         continue;
 
@@ -5168,6 +5430,7 @@ static usec_t epoch_or_infinity(void) {
 
 static int do_copy_files(Context *context, Partition *p, const char *root) {
         _cleanup_strv_free_ char **subvolumes = NULL;
+        _cleanup_free_ char **override_copy_files = NULL;
         int r;
 
         assert(p);
@@ -5177,11 +5440,17 @@ static int do_copy_files(Context *context, Partition *p, const char *root) {
         if (r < 0)
                 return r;
 
+        if (p->suppressing) {
+                r = shallow_join_strv(&override_copy_files, p->copy_files, p->suppressing->copy_files);
+                if (r < 0)
+                        return r;
+        }
+
         /* copy_tree_at() automatically copies the permissions of source directories to target directories if
          * it created them. However, the root directory is created by us, so we have to manually take care
          * that it is initialized. We use the first source directory targeting "/" as the metadata source for
          * the root directory. */
-        STRV_FOREACH_PAIR(source, target, p->copy_files) {
+        STRV_FOREACH_PAIR(source, target, override_copy_files ?: p->copy_files) {
                 _cleanup_close_ int rfd = -EBADF, sfd = -EBADF;
 
                 if (!path_equal(*target, "/"))
@@ -5202,7 +5471,7 @@ static int do_copy_files(Context *context, Partition *p, const char *root) {
                 break;
         }
 
-        STRV_FOREACH_PAIR(source, target, p->copy_files) {
+        STRV_FOREACH_PAIR(source, target, override_copy_files ?: p->copy_files) {
                 _cleanup_hashmap_free_ Hashmap *denylist = NULL;
                 _cleanup_set_free_ Set *subvolumes_by_source_inode = NULL;
                 _cleanup_close_ int sfd = -EBADF, pfd = -EBADF, tfd = -EBADF;
@@ -5320,6 +5589,7 @@ static int do_copy_files(Context *context, Partition *p, const char *root) {
 
 static int do_make_directories(Partition *p, const char *root) {
         _cleanup_strv_free_ char **subvolumes = NULL;
+        _cleanup_free_ char **override_dirs = NULL;
         int r;
 
         assert(p);
@@ -5329,7 +5599,13 @@ static int do_make_directories(Partition *p, const char *root) {
         if (r < 0)
                 return r;
 
-        STRV_FOREACH(d, p->make_directories) {
+        if (p->suppressing) {
+                r = shallow_join_strv(&override_dirs, p->make_directories, p->suppressing->make_directories);
+                if (r < 0)
+                        return r;
+        }
+
+        STRV_FOREACH(d, override_dirs ?: p->make_directories) {
                 r = mkdir_p_root_full(root, *d, UID_INVALID, GID_INVALID, 0755, epoch_or_infinity(), subvolumes);
                 if (r < 0)
                         return log_error_errno(r, "Failed to create directory '%s' in file system: %m", *d);
@@ -5377,6 +5653,12 @@ static int make_subvolumes_read_only(Partition *p, const char *root) {
                         return log_error_errno(r, "Failed to make subvolume '%s' read-only: %m", subvolume->path);
         }
 
+        if (p->suppressing) {
+                r = make_subvolumes_read_only(p->suppressing, root);
+                if (r < 0)
+                        return r;
+        }
+
         return 0;
 }
 
@@ -5496,6 +5778,38 @@ static int partition_populate_filesystem(Context *context, Partition *p, const c
         return 0;
 }
 
+static int append_btrfs_subvols(char ***l, OrderedHashmap *subvolumes, const char *default_subvolume) {
+        Subvolume *subvolume;
+        int r;
+
+        assert(l);
+
+        ORDERED_HASHMAP_FOREACH(subvolume, subvolumes) {
+                _cleanup_free_ char *s = NULL, *f = NULL;
+
+                s = strdup(subvolume->path);
+                if (!s)
+                        return log_oom();
+
+                f = subvolume_flags_to_string(subvolume->flags);
+                if (!f)
+                        return log_oom();
+
+                if (streq_ptr(subvolume->path, default_subvolume) &&
+                    !strextend_with_separator(&f, ",", "default"))
+                        return log_oom();
+
+                if (!isempty(f) && !strextend_with_separator(&s, ":", f))
+                        return log_oom();
+
+                r = strv_extend_many(l, "--subvol", s);
+                if (r < 0)
+                        return log_oom();
+        }
+
+        return 0;
+}
+
 static int finalize_extra_mkfs_options(const Partition *p, const char *root, char ***ret) {
         _cleanup_strv_free_ char **sv = NULL;
         int r;
@@ -5510,28 +5824,14 @@ static int finalize_extra_mkfs_options(const Partition *p, const char *root, cha
                                        p->format);
 
         if (partition_needs_populate(p) && root && streq(p->format, "btrfs")) {
-                Subvolume *subvolume;
-
-                ORDERED_HASHMAP_FOREACH(subvolume, p->subvolumes) {
-                        _cleanup_free_ char *s = NULL, *f = NULL;
-
-                        s = strdup(subvolume->path);
-                        if (!s)
-                                return log_oom();
-
-                        f = subvolume_flags_to_string(subvolume->flags);
-                        if (!f)
-                                return log_oom();
-
-                        if (streq_ptr(subvolume->path, p->default_subvolume) && !strextend_with_separator(&f, ",", "default"))
-                                return log_oom();
-
-                        if (!isempty(f) && !strextend_with_separator(&s, ":", f))
-                                return log_oom();
-
-                        r = strv_extend_many(&sv, "--subvol", s);
+                r = append_btrfs_subvols(&sv, p->subvolumes, p->default_subvolume);
                 if (r < 0)
-                                return log_oom();
+                        return r;
+
+                if (p->suppressing) {
+                        r = append_btrfs_subvols(&sv, p->suppressing->subvolumes, NULL);
+                        if (r < 0)
+                                return r;
                 }
         }
 
@@ -8524,7 +8824,7 @@ static int determine_auto_size(Context *c) {
         LIST_FOREACH(partitions, p, c->partitions) {
                 uint64_t m;
 
-                if (p->dropped)
+                if (p->dropped || PARTITION_SUPPRESSED(p))
                         continue;
 
                 m = partition_min_size_with_padding(c, p);
@@ -8756,13 +9056,36 @@ static int run(int argc, char *argv[]) {
                 if (context_allocate_partitions(context, &largest_free_area))
                         break; /* Success! */
 
-                if (!context_drop_or_foreignize_one_priority(context)) {
+                if (context_unmerge_and_allocate_partitions(context))
+                        break; /* We had to un-suppress a supplement or few, but still success! */
+
+                if (context_drop_or_foreignize_one_priority(context))
+                        continue; /* Still no luck. Let's drop a priority and try again. */
+
+                /* No more priorities left to drop. This configuration just doesn't fit on this disk... */
                 r = log_error_errno(SYNTHETIC_ERRNO(ENOSPC),
                                     "Can't fit requested partitions into available free space (%s), refusing.",
                                     FORMAT_BYTES(largest_free_area));
                 determine_auto_size(context);
                 return r;
         }
+
+        LIST_FOREACH(partitions, p, context->partitions) {
+                if (!p->supplement_for)
+                        continue;
+
+                if (PARTITION_SUPPRESSED(p)) {
+                        assert(!p->allocated_to_area);
+                        p->dropped = true;
+
+                        log_debug("Partition %s can be merged into %s, suppressing supplement.",
+                                  p->definition_path, p->supplement_for->definition_path);
+                } else if (PARTITION_EXISTS(p))
+                        log_info("Partition %s already exists on disk, using supplement verbatim.",
+                                 p->definition_path);
+                else
+                        log_info("Couldn't allocate partitions with %s merged into %s, using supplement verbatim.",
+                                 p->definition_path, p->supplement_for->definition_path);
         }
 
         /* Now assign free space according to the weight logic */

test/units/TEST-58-REPART.sh

@@ -29,6 +29,9 @@ if ! systemd-detect-virt --quiet --container; then
     udevadm control --log-level debug
 fi
 
+esp_guid=C12A7328-F81F-11D2-BA4B-00A0C93EC93B
+xbootldr_guid=BC13C2FF-59E6-4262-A352-B275FD6F7172
+
 machine="$(uname -m)"
 if [ "${machine}" = "x86_64" ]; then
     root_guid=4F68BCE3-E8CD-4DB1-96E7-FBCAF984B709
@@ -1429,6 +1432,82 @@ EOF
     systemd-dissect -U "$imgs/mnt"
 }
 
+testcase_fallback_partitions() {
+    local workdir image defs
+
+    workdir="$(mktemp --directory "/tmp/test-repart.fallback.XXXXXXXXXX")"
+    # shellcheck disable=SC2064
+    trap "rm -rf '${workdir:?}'" RETURN
+
+    image="$workdir/image.img"
+    defs="$workdir/defs"
+    mkdir "$defs"
+
+    tee "$defs/10-esp.conf" <<EOF
+[Partition]
+Type=esp
+Format=vfat
+SizeMinBytes=10M
+EOF
+
+    tee "$defs/20-xbootldr.conf" <<EOF
+[Partition]
+Type=xbootldr
+Format=vfat
+SizeMinBytes=100M
+SupplementFor=10-esp
+EOF
+
+    # Blank disk => big ESP should be created
+
+    systemd-repart --empty=create --size=auto --dry-run=no --definitions="$defs" "$image"
+
+    output=$(sfdisk -d "$image")
+    assert_in "${image}1 : start=        2048, size=      204800, type=${esp_guid}" "$output"
+    assert_not_in "${image}2" "$output"
+
+    # Disk with small ESP => ESP grows
+
+    sfdisk "$image" <<EOF
+label: gpt
+size=10M, type=${esp_guid}
+EOF
+
+    systemd-repart --dry-run=no --definitions="$defs" "$image"
+
+    output=$(sfdisk -d "$image")
+    assert_in "${image}1 : start=        2048, size=      204800, type=${esp_guid}" "$output"
+    assert_not_in "${image}2" "$output"
+
+    # Disk with small ESP that can't grow => XBOOTLDR created
+
+    truncate -s 150M "$image"
+    sfdisk "$image" <<EOF
+label: gpt
+size=10M, type=${esp_guid},
+size=10M, type=${root_guid},
+EOF
+
+    systemd-repart --dry-run=no --definitions="$defs" "$image"
+
+    output=$(sfdisk -d "$image")
+    assert_in "${image}1 : start=        2048, size=       20480, type=${esp_guid}" "$output"
+    assert_in "${image}3 : start=       43008, size=      264152, type=${xbootldr_guid}" "$output"
+
+    # Disk with existing XBOOTLDR partition => XBOOTLDR grows, small ESP created
+
+    sfdisk "$image" <<EOF
+label: gpt
+size=10M, type=${xbootldr_guid},
+EOF
+
+    systemd-repart --dry-run=no --definitions="$defs" "$image"
+
+    output=$(sfdisk -d "$image")
+    assert_in "${image}1 : start=        2048, size=      204800, type=${xbootldr_guid}" "$output"
+    assert_in "${image}2 : start=      206848, size=      100312, type=${esp_guid}" "$output"
+}
+
 OFFLINE="yes"
 run_testcases