3 files changed, 51 insertions, 4 deletions

diff --git a/src/bin/pg_basebackup/receivelog.c b/src/bin/pg_basebackup/receivelog.c
index f4151351727..8511e57cf7d 100644
--- a/src/bin/pg_basebackup/receivelog.c
+++ b/src/bin/pg_basebackup/receivelog.c
@@ -132,8 +132,11 @@ open_walfile(StreamCtl *stream, XLogRecPtr startpoint)
 			}
 
 			/* fsync file in case of a previous crash */
-			if (!stream->walmethod->sync(f))
+			if (stream->walmethod->sync(f) != 0)
 			{
+				fprintf(stderr,
+						_("%s: could not sync existing transaction log file \"%s\": %s\n"),
+						progname, fn, stream->walmethod->getlasterror());
 				stream->walmethod->close(f, CLOSE_UNLINK);
 				return false;
 			}
diff --git a/src/bin/pg_basebackup/walmethods.c b/src/bin/pg_basebackup/walmethods.c
index d9ad596bf06..d4de8ddcf78 100644
--- a/src/bin/pg_basebackup/walmethods.c
+++ b/src/bin/pg_basebackup/walmethods.c
@@ -61,7 +61,7 @@ typedef struct DirectoryMethodFile
 #endif
 }	DirectoryMethodFile;
 
-static char *
+static const char *
 dir_getlasterror(void)
 {
 	/* Directory method always sets errno, so just use strerror */
@@ -406,7 +406,7 @@ static TarMethodData *tar_data = NULL;
 #define tar_clear_error() tar_data->lasterror[0] = '\0'
 #define tar_set_error(msg) strlcpy(tar_data->lasterror, msg, sizeof(tar_data->lasterror))
 
-static char *
+static const char *
 tar_getlasterror(void)
 {
 	/*
diff --git a/src/bin/pg_basebackup/walmethods.h b/src/bin/pg_basebackup/walmethods.h
index 8d679dab615..35a280613f2 100644
--- a/src/bin/pg_basebackup/walmethods.h
+++ b/src/bin/pg_basebackup/walmethods.h
@@ -19,19 +19,63 @@ typedef enum
 	CLOSE_NO_RENAME
 }	WalCloseMethod;
 
+/*
+ * A WalWriteMethod structure represents the different methods used
+ * to write the streaming WAL as it's received.
+ *
+ * All methods that have a failure return indicator will set state
+ * allowing the getlasterror() method to return a suitable message.
+ * Commonly, errno is this state (or part of it); so callers must take
+ * care not to clobber errno between a failed method call and use of
+ * getlasterror() to retrieve the message.
+ */
 typedef struct WalWriteMethod WalWriteMethod;
 struct WalWriteMethod
 {
+	/*
+	 * Open a target file. Returns Walfile, or NULL if open failed. If a temp
+	 * suffix is specified, a file with that name will be opened, and then
+	 * automatically renamed in close(). If pad_to_size is specified, the file
+	 * will be padded with NUL up to that size, if supported by the Walmethod.
+	 */
 	Walfile(*open_for_write) (const char *pathname, const char *temp_suffix, size_t pad_to_size);
+
+	/*
+	 * Close an open Walfile, using one or more methods for handling automatic
+	 * unlinking etc. Returns 0 on success, other values for error.
+	 */
 	int			(*close) (Walfile f, WalCloseMethod method);
+
+	/* Check if a file exist */
 	bool		(*existsfile) (const char *pathname);
+
+	/* Return the size of a file, or -1 on failure. */
 	ssize_t		(*get_file_size) (const char *pathname);
 
+	/*
+	 * Write count number of bytes to the file, and return the number of bytes
+	 * actually written or -1 for error.
+	 */
 	ssize_t		(*write) (Walfile f, const void *buf, size_t count);
+
+	/* Return the current position in a file or -1 on error */
 	off_t		(*get_current_pos) (Walfile f);
+
+	/*
+	 * fsync the contents of the specified file. Returns 0 on success.
+	 */
 	int			(*sync) (Walfile f);
+
+	/*
+	 * Clean up the Walmethod, closing any shared resources. For methods like
+	 * tar, this includes writing updated headers. Returns true if the
+	 * close/write/sync of shared resources succeeded, otherwise returns false
+	 * (but the resources are still closed).
+	 */
 	bool		(*finish) (void);
-	char	   *(*getlasterror) (void);
+
+	/* Return a text for the last error in this Walfile */
+	const char *(*getlasterror) (void);
 };
 
 /*