repart: Fix growing the partition preceding a FreeArea from leftover space

The "Donate to preceding partition" logic is dead code since commit
19903a4 ("repart: split out context_grow_partition_one()").
context_grow_partition_one() gets passed a free area and a partition, and it has
an early-return check to ensure the partition it got passed belongs to the free
area it got passed. That means we compare the FreeArea a to the FreeArea
a->after->allocated_to_area, which always yields FALSE.

Fix the behavior of donating any left over space to the preceding partition
by adding that partition to the loop below (and relying on the partitions list
being ordered according to physical partition offsets).

Since this behavior is not that easy to trigger, mention how to trigger it in a
comment, and add a test for it as well.

src/repart/repart.c

@@ -1652,23 +1652,14 @@ static bool context_grow_partitions_phase(
         return !try_again;
 }
 
-static void context_grow_partition_one(Context *context, FreeArea *a, Partition *p, uint64_t *span) {
+static void context_grow_partition_one(Context *context, Partition *p, uint64_t *span) {
         uint64_t m;
 
         assert(context);
-        assert(a);
         assert(p);
         assert(span);
 
-        if (*span == 0)
-                return;
-
-        if (p->allocated_to_area != a)
-                return;
-
-        if (PARTITION_IS_FOREIGN(p))
-                return;
-
+        assert(*span > 0);
         assert(p->new_size != UINT64_MAX);
 
         /* Calculate new size and align. */
@@ -1708,15 +1699,15 @@ static int context_grow_partitions_on_free_area(Context *context, FreeArea *a) {
                 if (context_grow_partitions_phase(context, a, phase, &span, &weight_sum))
                         phase++; /* go to the next phase */
 
-        /* We still have space left over? Donate to preceding partition if we have one */
-        if (span > 0 && a->after)
-                context_grow_partition_one(context, a, a->after, &span);
 
-        /* What? Even still some space left (maybe because there was no preceding partition, or it had a
-         * size limit), then let's donate it to whoever wants it. */
+        /* What? Even still some space left (because one partition had max_size < share
+         * and another had min_size > share), then let's donate it to whoever wants it. */
         if (span > 0)
                 LIST_FOREACH(partitions, p, context->partitions) {
-                        context_grow_partition_one(context, a, p, &span);
+                        if (p->allocated_to_area != a && p->padding_area != a)
+                                continue;
+
+                        context_grow_partition_one(context, p, &span);
                         if (span == 0)
                                 break;
                 }

test/units/TEST-58-REPART.sh

@@ -2703,6 +2703,92 @@ EOF
     assert_in "$imgs/gap.img3 : start=       22528, size=       20480, type=$usr_guid, uuid=$usr_uuid, name=\"part-b\"" "$output"
 }
 
+testcase_distribute_leftover_space() {
+    local defs imgs output
+
+    defs="$(mktemp --directory "/tmp/test-repart.defs.XXXXXXXXXX")"
+    imgs="$(mktemp --directory "/var/tmp/test-repart.imgs.XXXXXXXXXX")"
+    # shellcheck disable=SC2064
+    trap "rm -rf '$defs' '$imgs'" RETURN
+    chmod 0755 "$defs"
+
+    echo "*** Left over space after distributing according to weights should still be assigned ***"
+
+    tee "$defs/01-esp.conf" <<EOF
+[Partition]
+Type=esp
+Weight=1
+SizeMinBytes=5M
+EOF
+
+    tee "$defs/02-usr.conf" <<EOF
+[Partition]
+Type=usr-${architecture}
+Weight=1000
+SizeMinBytes=1M
+SizeMaxBytes=2M
+EOF
+
+    truncate -s 20M "$imgs/leftover.img"
+    systemd-repart --offline="$OFFLINE" \
+                   --definitions="$defs" \
+                   --seed="$seed" \
+                   --dry-run=no \
+                   --empty=allow \
+                   "$imgs/leftover.img"
+
+    output=$(sfdisk --dump "$imgs/leftover.img")
+
+    # esp should get all the leftover space
+    assert_in "$imgs/leftover.img1 : start=        2048, size=       34776, type=$esp_guid," "$output"
+    assert_in "$imgs/leftover.img2 : start=       36824, size=        4096, type=$usr_guid," "$output"
+
+    rm "$defs/01-esp.conf"
+    rm "$defs/02-usr.conf"
+    rm "$imgs/leftover.img"
+
+    tee "$defs/01-xbootldr.conf" <<EOF
+[Partition]
+Type=xbootldr
+SizeMaxBytes=10M
+EOF
+
+    tee "$defs/02-usr.conf" <<EOF
+[Partition]
+Type=usr-${architecture}
+Weight=1000
+SizeMinBytes=1M
+SizeMaxBytes=2M
+EOF
+
+    tee "$defs/03-esp.conf" <<EOF
+[Partition]
+Type=esp
+Weight=1
+SizeMinBytes=5M
+EOF
+
+    truncate -s 25M "$imgs/leftover2.img"
+    sfdisk "$imgs/leftover2.img" <<EOF
+label: gpt
+size=3M, type=${xbootldr_guid},
+EOF
+
+    systemd-repart --offline="$OFFLINE" \
+                   --definitions="$defs" \
+                   --seed="$seed" \
+                   --dry-run=no \
+                   --empty=allow \
+                   "$imgs/leftover2.img"
+
+    output=$(sfdisk --dump "$imgs/leftover2.img")
+
+    # xbootldr should get the leftover space and grow to 10M, then esp should get the rest
+    assert_in "$imgs/leftover2.img1 : start=        2048, size=       20480, type=$xbootldr_guid," "$output"
+    assert_in "$imgs/leftover2.img2 : start=       22528, size=        4096, type=$usr_guid," "$output"
+    assert_in "$imgs/leftover2.img3 : start=       26624, size=       24536, type=$esp_guid," "$output"
+}
+
 OFFLINE="yes"
 run_testcases