Create read_or_die utility routine.

Like write_or_die read_or_die reads the entire length requested
or it kills the current process with a die call.
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

cache.h

@@ -428,6 +428,7 @@ extern char *git_commit_encoding;
 extern char *git_log_output_encoding;
 
 extern int copy_fd(int ifd, int ofd);
+extern void read_or_die(int fd, void *buf, size_t count);
 extern void write_or_die(int fd, const void *buf, size_t count);
 extern int write_or_whine(int fd, const void *buf, size_t count, const char *msg);
 

write_or_die.c

 #include "cache.h"
 
+void read_or_die(int fd, void *buf, size_t count)
+{
+	char *p = buf;
+	ssize_t loaded;
+
+	while (count > 0) {
+		loaded = xread(fd, p, count);
+		if (loaded == 0)
+			die("unexpected end of file");
+		else if (loaded < 0)
+			die("read error (%s)", strerror(errno));
+		count -= loaded;
+		p += loaded;
+	}
+}
+
 void write_or_die(int fd, const void *buf, size_t count)
 {
 	const char *p = buf;