From cfb758b6d9c1df58fb1dfd5d3f6e70393fb17869 Mon Sep 17 00:00:00 2001
From: Magnus Hagander <magnus@hagander.net>
Date: Fri, 18 May 2018 17:52:18 +0200
Subject: [PATCH] Fix error message on short read of pg_control

Instead of saying "error: success", indicate that we got a working read
but it was too short.
---
 src/backend/access/transam/xlog.c | 16 +++++++++----
 src/common/controldata_utils.c    | 37 ++++++++++++++++++++++---------
 2 files changed, 39 insertions(+), 14 deletions(-)

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 04bfac7485..adbd6a2126 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -4486,6 +4486,7 @@ ReadControlFile(void)
 	pg_crc32c	crc;
 	int			fd;
 	static char wal_segsz_str[20];
+	int			r;
 
 	/*
 	 * Read data...
@@ -4499,10 +4500,17 @@ ReadControlFile(void)
 						XLOG_CONTROL_FILE)));
 
 	pgstat_report_wait_start(WAIT_EVENT_CONTROL_FILE_READ);
-	if (read(fd, ControlFile, sizeof(ControlFileData)) != sizeof(ControlFileData))
-		ereport(PANIC,
-				(errcode_for_file_access(),
-				 errmsg("could not read from control file: %m")));
+	r = read(fd, ControlFile, sizeof(ControlFileData));
+	if (r != sizeof(ControlFileData))
+	{
+		if (r < 0)
+			ereport(PANIC,
+					(errcode_for_file_access(),
+					 errmsg("could not read from control file: %m")));
+		else
+			ereport(PANIC,
+					 (errmsg("could not read from control file: read %d bytes, expected %d", r, (int) sizeof(ControlFileData))));
+	}
 	pgstat_report_wait_end();
 
 	close(fd);
diff --git a/src/common/controldata_utils.c b/src/common/controldata_utils.c
index f12a188856..78a9f0ce62 100644
--- a/src/common/controldata_utils.c
+++ b/src/common/controldata_utils.c
@@ -44,6 +44,7 @@ get_controlfile(const char *DataDir, const char *progname, bool *crc_ok_p)
 	int			fd;
 	char		ControlFilePath[MAXPGPATH];
 	pg_crc32c	crc;
+	int			r;
 
 	AssertArg(crc_ok_p);
 
@@ -64,18 +65,34 @@ get_controlfile(const char *DataDir, const char *progname, bool *crc_ok_p)
 	}
 #endif
 
-	if (read(fd, ControlFile, sizeof(ControlFileData)) != sizeof(ControlFileData))
-#ifndef FRONTEND
-		ereport(ERROR,
-				(errcode_for_file_access(),
-				 errmsg("could not read file \"%s\": %m", ControlFilePath)));
-#else
+	r = read(fd, ControlFile, sizeof(ControlFileData));
+	if (r != sizeof(ControlFileData))
 	{
-		fprintf(stderr, _("%s: could not read file \"%s\": %s\n"),
-				progname, ControlFilePath, strerror(errno));
-		exit(EXIT_FAILURE);
-	}
+		if (r < 0)
+#ifndef FRONTEND
+			ereport(ERROR,
+					(errcode_for_file_access(),
+					 errmsg("could not read file \"%s\": %m", ControlFilePath)));
+#else
+		{
+			fprintf(stderr, _("%s: could not read file \"%s\": %s\n"),
+					progname, ControlFilePath, strerror(errno));
+			exit(EXIT_FAILURE);
+		}
 #endif
+		else
+#ifndef FRONTEND
+			ereport(ERROR,
+					(errmsg("could not read file \"%s\": read %d bytes, expected %d",
+							ControlFilePath, r, (int) sizeof(ControlFileData))));
+#else
+		{
+			fprintf(stderr, _("%s: could not read file \"%s\": read %d bytes, expected %d\n"),
+					progname, ControlFilePath, r, (int) sizeof(ControlFileData));
+			exit(EXIT_FAILURE);
+		}
+#endif
+	}
 
 	close(fd);