Change relpath() et al to return path by value

For AIO, and also some other recent patches, we need the ability to call
relpath() in a critical section. Until now that was not feasible, as it
allocated memory.

The fact that relpath() allocated memory also made it awkward to use in log
messages because we had to take care to free the memory afterwards. Which we
e.g. didn't do for when zeroing out an invalid buffer.

We discussed other solutions, e.g. filling a pre-allocated buffer that's
passed to relpath(), but they all came with plenty downsides or were larger
projects. The easiest fix seems to be to make relpath() return the path by
value.

To be able to return the path by value we need to determine the maximum length
of a relation path. This patch adds a long #define that computes the exact
maximum, which is verified to be correct in a regression test.

As this change the signature of relpath(), extensions using it will need to
adapt their code. We discussed leaving a backward-compat shim in place, but
decided it's not worth it given the use of relpath() doesn't seem widespread.

Discussion: https://postgr.es/m/xeri5mla4b5syjd5a25nok5iez2kr3bm26j2qn4u7okzof2bmf@kwdh2vf7npra

2 files changed, 4 insertions, 6 deletions

diff --git a/src/backend/catalog/catalog.c b/src/backend/catalog/catalog.c
index 05ebe248f12..8436e312d4d 100644
--- a/src/backend/catalog/catalog.c
+++ b/src/backend/catalog/catalog.c
@@ -528,7 +528,7 @@ RelFileNumber
 GetNewRelFileNumber(Oid reltablespace, Relation pg_class, char relpersistence)
 {
 	RelFileLocatorBackend rlocator;
-	char	   *rpath;
+	RelPathStr	rpath;
 	bool		collides;
 	ProcNumber	procNumber;
 
@@ -580,7 +580,7 @@ GetNewRelFileNumber(Oid reltablespace, Relation pg_class, char relpersistence)
 		/* Check for existing file of same name */
 		rpath = relpath(rlocator, MAIN_FORKNUM);
 
-		if (access(rpath, F_OK) == 0)
+		if (access(rpath.str, F_OK) == 0)
 		{
 			/* definite collision */
 			collides = true;
@@ -596,8 +596,6 @@ GetNewRelFileNumber(Oid reltablespace, Relation pg_class, char relpersistence)
 			 */
 			collides = false;
 		}
-
-		pfree(rpath);
 	} while (collides);
 
 	return rlocator.locator.relNumber;
diff --git a/src/backend/catalog/storage.c b/src/backend/catalog/storage.c
index 690b1dbc6ee..624ed41bbf3 100644
--- a/src/backend/catalog/storage.c
+++ b/src/backend/catalog/storage.c
@@ -524,14 +524,14 @@ RelationCopyStorage(SMgrRelation src, SMgrRelation dst,
 			 * (errcontext callbacks shouldn't be risking any such thing, but
 			 * people have been known to forget that rule.)
 			 */
-			char	   *relpath = relpathbackend(src->smgr_rlocator.locator,
+			RelPathStr	relpath = relpathbackend(src->smgr_rlocator.locator,
 												 src->smgr_rlocator.backend,
 												 forkNum);
 
 			ereport(ERROR,
 					(errcode(ERRCODE_DATA_CORRUPTED),
 					 errmsg("invalid page in block %u of relation %s",
-							blkno, relpath)));
+							blkno, relpath.str)));
 		}
 
 		/*