pipewire: mem: pw_memblock_map(): fix pointer when reusing mapping

Previously the pointer was determined as follows:

  mm->this.ptr = SPA_PTROFF(m->ptr, range.start, void);

however, when `pw_map_range` is calculated, `pw_map_range::start` is the offset
from the beginning of the first page, starting at `pw_map_range::offset`.

This works correctly if `memblock_map()` runs because that will map the file
with expected offset, so using `range.start` is correct.

However, when a mapping is reused (i.e. `memblock_find_mapping()`) finds something,
then `range.start` is not necessarily correct. Consider the following example:

  * page size is 10
  * one memblock with size 20 (2 pages)
  * the applications wants to mappings:
    * (offset=5,size=10)
    * (offset=15,size=5)

After the first request from the application, a `mapping` object is created
that covers the first two pages of the memblock: offset=0 and size=20. During
the second request, the calculated `pw_map_range` is as follows:

  { start = 5, offset = 10, size = 10 }

and the only previously created mapping is reused since (0 <= 5) and (10 <= 20). When
the pointer of the mapping is adjusted afterwards it will be incorrect since `m->ptr`
points to byte 0 on page 0 (instead of byte 0 on page 1 -- that is assumed). Thereforce
the two will unexpectedly overlap.

Fix that by using `offset - m->offset` when adjusting the mapping's pointer. Also move
the `range` variable into a smaller scope because it only makes sense there. And add
a test that check the above previously incorrect case.

Fixes: 2caf81c97c ("mem: improve memory handling")
Fixes #4884

test/test-mempool.c

@@ -0,0 +1,49 @@
+/* PipeWire */
+/* SPDX-FileCopyrightText: Copyright © 2025 PipeWire authors */
+/* SPDX-License-Identifier: MIT */
+
+#include <unistd.h>
+
+#include <pipewire/mem.h>
+#include <spa/buffer/buffer.h>
+
+#include "pwtest.h"
+
+PWTEST(mempool_issue4884)
+{
+	/*
+	 * See https://gitlab.freedesktop.org/pipewire/pipewire/-/issues/4884. This
+	 * test checks if the offset is correctly applied when a mapping is reused.
+	 */
+
+	long page_size = sysconf(_SC_PAGESIZE);
+	pwtest_errno_ok(page_size);
+	pwtest_int_ge(page_size, 8);
+
+	struct pw_mempool *p = pw_mempool_new(NULL);
+	pwtest_ptr_notnull(p);
+
+	struct pw_memblock *b = pw_mempool_alloc(p, PW_MEMBLOCK_FLAG_READWRITE, SPA_DATA_MemFd, 2 * page_size);
+	pwtest_ptr_notnull(b);
+
+	struct pw_memmap *m1 = pw_mempool_map_id(p, b->id, PW_MEMMAP_FLAG_READWRITE, page_size / 2, page_size, NULL);
+	pwtest_ptr_notnull(m1);
+	pwtest_ptr_eq(m1->block, b);
+
+	struct pw_memmap *m2 = pw_mempool_map_id(p, b->id, PW_MEMMAP_FLAG_READWRITE, 3 * page_size / 2, page_size / 2, NULL);
+	pwtest_ptr_notnull(m2);
+	pwtest_ptr_eq(m2->block, b);
+
+	pwtest_int_eq(SPA_PTRDIFF(m2->ptr, m1->ptr), page_size);
+
+	pw_mempool_destroy(p);
+
+	return PWTEST_PASS;
+}
+
+PWTEST_SUITE(pw_mempool)
+{
+	pwtest_add(mempool_issue4884, PWTEST_NOARG);
+
+	return PWTEST_PASS;
+}