Fix unsafe references to errno within error messaging logic.

Various places were supposing that errno could be expected to hold still
within an ereport() nest or similar contexts.  This isn't true necessarily,
though in some cases it accidentally failed to fail depending on how the
compiler chanced to order the subexpressions.  This class of thinko
explains recent reports of odd failures on clang-built versions, typically
missing or inappropriate HINT fields in messages.

Problem identified by Christian Kruse, who also submitted the patch this
commit is based on.  (I fixed a few issues in his patch and found a couple
of additional places with the same disease.)

Back-patch as appropriate to all supported branches.

src/backend/commands/tablespace.c

@@ -770,10 +770,14 @@ remove_symlink:
 	else
 	{
 		if (unlink(linkloc) < 0)
-			ereport(redo ? LOG : (errno == ENOENT ? WARNING : ERROR),
+		{
+			int			saved_errno = errno;
+
+			ereport(redo ? LOG : (saved_errno == ENOENT ? WARNING : ERROR),
 					(errcode_for_file_access(),
 					 errmsg("could not remove symbolic link \"%s\": %m",
 							linkloc)));
+		}
 	}
 
 	pfree(linkloc_with_version_dir);

src/backend/port/sysv_sema.c

@@ -91,15 +91,17 @@ InternalIpcSemaphoreCreate(IpcSemaphoreKey semKey, int numSems)
 
 	if (semId < 0)
 	{
+		int			saved_errno = errno;
+
 		/*
 		 * Fail quietly if error indicates a collision with existing set. One
 		 * would expect EEXIST, given that we said IPC_EXCL, but perhaps we
 		 * could get a permission violation instead?  Also, EIDRM might occur
 		 * if an old set is slated for destruction but not gone yet.
 		 */
-		if (errno == EEXIST || errno == EACCES
+		if (saved_errno == EEXIST || saved_errno == EACCES
 #ifdef EIDRM
-			|| errno == EIDRM
+			|| saved_errno == EIDRM
 #endif
 			)
 			return -1;
@@ -112,7 +114,7 @@ InternalIpcSemaphoreCreate(IpcSemaphoreKey semKey, int numSems)
 				 errdetail("Failed system call was semget(%lu, %d, 0%o).",
 						   (unsigned long) semKey, numSems,
 						   IPC_CREAT | IPC_EXCL | IPCProtection),
-				 (errno == ENOSPC) ?
+				 (saved_errno == ENOSPC) ?
 				 errhint("This error does *not* mean that you have run out of disk space.  "
 		  "It occurs when either the system limit for the maximum number of "
 			 "semaphore sets (SEMMNI), or the system wide maximum number of "
@@ -136,13 +138,17 @@ IpcSemaphoreInitialize(IpcSemaphoreId semId, int semNum, int value)
 
 	semun.val = value;
 	if (semctl(semId, semNum, SETVAL, semun) < 0)
+	{
+		int			saved_errno = errno;
+
 		ereport(FATAL,
 				(errmsg_internal("semctl(%d, %d, SETVAL, %d) failed: %m",
 								 semId, semNum, value),
-				 (errno == ERANGE) ?
+				 (saved_errno == ERANGE) ?
 				 errhint("You possibly need to raise your kernel's SEMVMX value to be at least "
 				  "%d.  Look into the PostgreSQL documentation for details.",
 						 value) : 0));
+	}
 }
 
 /*

src/backend/port/sysv_shmem.c

@@ -96,15 +96,17 @@ InternalIpcMemoryCreate(IpcMemoryKey memKey, Size size)
 
 	if (shmid < 0)
 	{
+		int			shmget_errno = errno;
+
 		/*
 		 * Fail quietly if error indicates a collision with existing segment.
 		 * One would expect EEXIST, given that we said IPC_EXCL, but perhaps
 		 * we could get a permission violation instead?  Also, EIDRM might
 		 * occur if an old seg is slated for destruction but not gone yet.
 		 */
-		if (errno == EEXIST || errno == EACCES
+		if (shmget_errno == EEXIST || shmget_errno == EACCES
 #ifdef EIDRM
-			|| errno == EIDRM
+			|| shmget_errno == EIDRM
 #endif
 			)
 			return NULL;
@@ -118,10 +120,8 @@ InternalIpcMemoryCreate(IpcMemoryKey memKey, Size size)
 		 * against SHMMIN in the preexisting-segment case, so we will not get
 		 * EINVAL a second time if there is such a segment.
 		 */
-		if (errno == EINVAL)
+		if (shmget_errno == EINVAL)
 		{
-			int			save_errno = errno;
-
 			shmid = shmget(memKey, 0, IPC_CREAT | IPC_EXCL | IPCProtection);
 
 			if (shmid < 0)
@@ -147,8 +147,6 @@ InternalIpcMemoryCreate(IpcMemoryKey memKey, Size size)
 					elog(LOG, "shmctl(%d, %d, 0) failed: %m",
 						 (int) shmid, IPC_RMID);
 			}
-
-			errno = save_errno;
 		}
 
 		/*
@@ -160,25 +158,26 @@ InternalIpcMemoryCreate(IpcMemoryKey memKey, Size size)
 		 * it should be.  SHMMNI violation is ENOSPC, per spec.  Just plain
 		 * not-enough-RAM is ENOMEM.
 		 */
+		errno = shmget_errno;
 		ereport(FATAL,
 				(errmsg("could not create shared memory segment: %m"),
 		  errdetail("Failed system call was shmget(key=%lu, size=%lu, 0%o).",
 					(unsigned long) memKey, (unsigned long) size,
 					IPC_CREAT | IPC_EXCL | IPCProtection),
-				 (errno == EINVAL) ?
+				 (shmget_errno == EINVAL) ?
 				 errhint("This error usually means that PostgreSQL's request for a shared memory "
 		 "segment exceeded your kernel's SHMMAX parameter, or possibly that "
 						 "it is less than "
 						 "your kernel's SHMMIN parameter.\n"
 		"The PostgreSQL documentation contains more information about shared "
 						 "memory configuration.") : 0,
-				 (errno == ENOMEM) ?
+				 (shmget_errno == ENOMEM) ?
 				 errhint("This error usually means that PostgreSQL's request for a shared "
 						 "memory segment exceeded your kernel's SHMALL parameter.  You might need "
 						 "to reconfigure the kernel with larger SHMALL.\n"
 		"The PostgreSQL documentation contains more information about shared "
 						 "memory configuration.") : 0,
-				 (errno == ENOSPC) ?
+				 (shmget_errno == ENOSPC) ?
 				 errhint("This error does *not* mean that you have run out of disk space.  "
 						 "It occurs either if all available shared memory IDs have been taken, "
 						 "in which case you need to raise the SHMMNI parameter in your kernel, "
@@ -413,9 +412,12 @@ PGSharedMemoryCreate(Size size, bool makePrivate, int port)
 		AnonymousShmem = mmap(NULL, size, PROT_READ | PROT_WRITE, PG_MMAP_FLAGS,
 							  -1, 0);
 		if (AnonymousShmem == MAP_FAILED)
+		{
+			int		saved_errno = errno;
+
 			ereport(FATAL,
 					(errmsg("could not map anonymous shared memory: %m"),
-					 (errno == ENOMEM) ?
+					 (saved_errno == ENOMEM) ?
 				errhint("This error usually means that PostgreSQL's request "
 					 "for a shared memory segment exceeded available memory "
 					  "or swap space. To reduce the request size (currently "
@@ -423,6 +425,7 @@ PGSharedMemoryCreate(Size size, bool makePrivate, int port)
 						"perhaps by reducing shared_buffers or "
 						"max_connections.",
 						(unsigned long) size) : 0));
+		}
 		AnonymousShmemSize = size;
 
 		/* Now we need only allocate a minimal-sized SysV shmem block. */