Address set of issues with errno handling

System calls mixed up in error code paths are causing two issues which
several code paths have not correctly handled:
1) For write() calls, sometimes the system may return less bytes than
what has been written without errno being set.  Some paths were careful
enough to consider that case, and assumed that errno should be set to
ENOSPC, other calls missed that.
2) errno generated by a system call is overwritten by other system calls
which may succeed once an error code path is taken, causing what is
reported to the user to be incorrect.

This patch uses the brute-force approach of correcting all those code
paths.  Some refactoring could happen in the future, but this is let as
future work, which is not targeted for back-branches anyway.

Author: Michael Paquier
Reviewed-by: Ashutosh Sharma
Discussion: https://postgr.es/m/20180622061535.GD5215@paquier.xyz

1 files changed, 32 insertions, 2 deletions

diff --git a/src/bin/pg_basebackup/walmethods.c b/src/bin/pg_basebackup/walmethods.c
index 02d368b2427..a48aacf7661 100644
--- a/src/bin/pg_basebackup/walmethods.c
+++ b/src/bin/pg_basebackup/walmethods.c
@@ -127,7 +127,11 @@ dir_open_for_write(const char *pathname, const char *temp_suffix, size_t pad_to_
 
 				pg_free(zerobuf);
 				close(fd);
-				errno = save_errno;
+
+				/*
+				 * If write didn't set errno, assume problem is no disk space.
+				 */
+				errno = save_errno ? save_errno : ENOSPC;
 				return NULL;
 			}
 		}
@@ -441,7 +445,14 @@ tar_write_compressed_data(void *buf, size_t count, bool flush)
 			size_t		len = ZLIB_OUT_SIZE - tar_data->zp->avail_out;
 
 			if (write(tar_data->fd, tar_data->zlibOut, len) != len)
+			{
+				/*
+				 * If write didn't set errno, assume problem is no disk space.
+				 */
+				if (errno == 0)
+					errno = ENOSPC;
 				return false;
+			}
 
 			tar_data->zp->next_out = tar_data->zlibOut;
 			tar_data->zp->avail_out = ZLIB_OUT_SIZE;
@@ -621,7 +632,8 @@ tar_open_for_write(const char *pathname, const char *temp_suffix, size_t pad_to_
 			save_errno = errno;
 			pg_free(tar_data->currentfile);
 			tar_data->currentfile = NULL;
-			errno = save_errno;
+			/* if write didn't set errno, assume problem is no disk space */
+			errno = save_errno ? save_errno : ENOSPC;
 			return NULL;
 		}
 	}
@@ -816,7 +828,12 @@ tar_close(Walfile f, WalCloseMethod method)
 	if (!tar_data->compression)
 	{
 		if (write(tar_data->fd, tf->header, 512) != 512)
+		{
+			/* if write didn't set errno, assume problem is no disk space */
+			if (errno == 0)
+				errno = ENOSPC;
 			return -1;
+		}
 	}
 #ifdef HAVE_LIBZ
 	else
@@ -882,7 +899,12 @@ tar_finish(void)
 	if (!tar_data->compression)
 	{
 		if (write(tar_data->fd, zerobuf, sizeof(zerobuf)) != sizeof(zerobuf))
+		{
+			/* if write didn't set errno, assume problem is no disk space */
+			if (errno == 0)
+				errno = ENOSPC;
 			return false;
+		}
 	}
 #ifdef HAVE_LIBZ
 	else
@@ -909,7 +931,15 @@ tar_finish(void)
 				size_t		len = ZLIB_OUT_SIZE - tar_data->zp->avail_out;
 
 				if (write(tar_data->fd, tar_data->zlibOut, len) != len)
+				{
+					/*
+					 * If write didn't set errno, assume problem is no disk
+					 * space.
+					 */
+					if (errno == 0)
+						errno = ENOSPC;
 					return false;
+				}
 			}
 			if (r == Z_STREAM_END)
 				break;