sed: Try hard links to make -i target available continually.

When creating a backup file, sed renamed the original before renaming the
changed copy into place, leading to a short time when no file with the
original name was present (usually only visible on SMP systems). Try
creating the backup file using a hard link instead, avoiding this problem.
If creating the hard link fails for any reason, fall back to the old rename
method.

When not creating a backup file, sed already renamed the changed copy onto
the original. This remains unchanged.

I am not adding the suppression of redundant fchown/fchmod to this commit,
because FreeBSD appears to check this in the kernel (for msdosfs at least).

PR:		bin/153261
Submitted by:	Pedro F. Giffuni
Reviewed by:	dds (older version)
Obtained from:	Illumos
MFC after:	2 weeks

1 files changed, 21 insertions, 4 deletions

diff --git a/usr.bin/sed/main.c b/usr.bin/sed/main.c
index 8d4fe95..049d2ea 100644
--- a/usr.bin/sed/main.c
+++ b/usr.bin/sed/main.c
@@ -338,18 +338,35 @@ mf_fgets(SPACE *sp, enum e_spflag spflag)
 		if (infile != NULL) {
 			fclose(infile);
 			if (*oldfname != '\0') {
-				if (rename(fname, oldfname) != 0) {
+				/* if there was a backup file, remove it */
+				unlink(oldfname);
+				/*
+				 * Backup the original.  Note that hard links
+				 * are not supported on all filesystems.
+				 */
+				if ((link(fname, oldfname) != 0) &&
+				   (rename(fname, oldfname) != 0)) {
 					warn("rename()");
-					unlink(tmpfname);
+					if (*tmpfname)
+						unlink(tmpfname);
 					exit(1);
 				}
 				*oldfname = '\0';
 			}
 			if (*tmpfname != '\0') {
 				if (outfile != NULL && outfile != stdout)
-					fclose(outfile);
+					if (fclose(outfile) != 0) {
+						warn("fclose()");
+						unlink(tmpfname);
+						exit(1);
+					}
 				outfile = NULL;
-				rename(tmpfname, fname);
+				if (rename(tmpfname, fname) != 0) {
+					/* this should not happen really! */
+					warn("rename()");
+					unlink(tmpfname);
+					exit(1);
+				}
 				*tmpfname = '\0';
 			}
 			outfname = NULL;