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

1 files changed, 42 insertions, 35 deletions

diff --git a/src/common/relpath.c b/src/common/relpath.c
index 186156a9e44..7dcf987afcd 100644
--- a/src/common/relpath.c
+++ b/src/common/relpath.c
@@ -132,17 +132,18 @@ GetDatabasePath(Oid dbOid, Oid spcOid)
 /*
  * GetRelationPath - construct path to a relation's file
  *
- * Result is a palloc'd string.
+ * The result is returned in-place as a struct, to make it suitable for use in
+ * critical sections etc.
  *
  * Note: ideally, procNumber would be declared as type ProcNumber, but
  * relpath.h would have to include a backend-only header to do that; doesn't
  * seem worth the trouble considering ProcNumber is just int anyway.
  */
-char *
+RelPathStr
 GetRelationPath(Oid dbOid, Oid spcOid, RelFileNumber relNumber,
 				int procNumber, ForkNumber forkNumber)
 {
-	char	   *path;
+	RelPathStr	rp;
 
 	if (spcOid == GLOBALTABLESPACE_OID)
 	{
@@ -150,10 +151,11 @@ GetRelationPath(Oid dbOid, Oid spcOid, RelFileNumber relNumber,
 		Assert(dbOid == 0);
 		Assert(procNumber == INVALID_PROC_NUMBER);
 		if (forkNumber != MAIN_FORKNUM)
-			path = psprintf("global/%u_%s",
-							relNumber, forkNames[forkNumber]);
+			sprintf(rp.str, "global/%u_%s",
+					relNumber, forkNames[forkNumber]);
 		else
-			path = psprintf("global/%u", relNumber);
+			sprintf(rp.str, "global/%u",
+					relNumber);
 	}
 	else if (spcOid == DEFAULTTABLESPACE_OID)
 	{
@@ -161,22 +163,24 @@ GetRelationPath(Oid dbOid, Oid spcOid, RelFileNumber relNumber,
 		if (procNumber == INVALID_PROC_NUMBER)
 		{
 			if (forkNumber != MAIN_FORKNUM)
-				path = psprintf("base/%u/%u_%s",
-								dbOid, relNumber,
-								forkNames[forkNumber]);
+			{
+				sprintf(rp.str, "base/%u/%u_%s",
+						dbOid, relNumber,
+						forkNames[forkNumber]);
+			}
 			else
-				path = psprintf("base/%u/%u",
-								dbOid, relNumber);
+				sprintf(rp.str, "base/%u/%u",
+						dbOid, relNumber);
 		}
 		else
 		{
 			if (forkNumber != MAIN_FORKNUM)
-				path = psprintf("base/%u/t%d_%u_%s",
-								dbOid, procNumber, relNumber,
-								forkNames[forkNumber]);
+				sprintf(rp.str, "base/%u/t%d_%u_%s",
+						dbOid, procNumber, relNumber,
+						forkNames[forkNumber]);
 			else
-				path = psprintf("base/%u/t%d_%u",
-								dbOid, procNumber, relNumber);
+				sprintf(rp.str, "base/%u/t%d_%u",
+						dbOid, procNumber, relNumber);
 		}
 	}
 	else
@@ -185,31 +189,34 @@ GetRelationPath(Oid dbOid, Oid spcOid, RelFileNumber relNumber,
 		if (procNumber == INVALID_PROC_NUMBER)
 		{
 			if (forkNumber != MAIN_FORKNUM)
-				path = psprintf("%s/%u/%s/%u/%u_%s",
-								PG_TBLSPC_DIR, spcOid,
-								TABLESPACE_VERSION_DIRECTORY,
-								dbOid, relNumber,
-								forkNames[forkNumber]);
+				sprintf(rp.str, "%s/%u/%s/%u/%u_%s",
+						PG_TBLSPC_DIR, spcOid,
+						TABLESPACE_VERSION_DIRECTORY,
+						dbOid, relNumber,
+						forkNames[forkNumber]);
 			else
-				path = psprintf("%s/%u/%s/%u/%u",
-								PG_TBLSPC_DIR, spcOid,
-								TABLESPACE_VERSION_DIRECTORY,
-								dbOid, relNumber);
+				sprintf(rp.str, "%s/%u/%s/%u/%u",
+						PG_TBLSPC_DIR, spcOid,
+						TABLESPACE_VERSION_DIRECTORY,
+						dbOid, relNumber);
 		}
 		else
 		{
 			if (forkNumber != MAIN_FORKNUM)
-				path = psprintf("%s/%u/%s/%u/t%d_%u_%s",
-								PG_TBLSPC_DIR, spcOid,
-								TABLESPACE_VERSION_DIRECTORY,
-								dbOid, procNumber, relNumber,
-								forkNames[forkNumber]);
+				sprintf(rp.str, "%s/%u/%s/%u/t%d_%u_%s",
+						PG_TBLSPC_DIR, spcOid,
+						TABLESPACE_VERSION_DIRECTORY,
+						dbOid, procNumber, relNumber,
+						forkNames[forkNumber]);
 			else
-				path = psprintf("%s/%u/%s/%u/t%d_%u",
-								PG_TBLSPC_DIR, spcOid,
-								TABLESPACE_VERSION_DIRECTORY,
-								dbOid, procNumber, relNumber);
+				sprintf(rp.str, "%s/%u/%s/%u/t%d_%u",
+						PG_TBLSPC_DIR, spcOid,
+						TABLESPACE_VERSION_DIRECTORY,
+						dbOid, procNumber, relNumber);
 		}
 	}
-	return path;
+
+	Assert(strnlen(rp.str, REL_PATH_STR_MAXLEN + 1) <= REL_PATH_STR_MAXLEN);
+
+	return rp;
 }