libdpkg: Switch updatefn handling to use varbuf snapshots

This uses the common varbuf snapshot and rollback pattern which
handles automatic memory reallocation if needed, and is less prone to
buffer handling mistakes.

lib/dpkg/dbmodify.c

@@ -57,7 +57,8 @@ static FILE *importanttmp;
 static int nextupdate;
 static char *updatesdir;
 static int updateslength;
-static char *updatefnbuf, *updatefnrest;
+static struct varbuf updatefn;
+static struct varbuf_state updatefn_state;
 static struct varbuf uvb;
 
 static int ulist_select(const struct dirent *de) {
@@ -82,7 +83,6 @@ static void cleanupdates(void) {
 
   parsedb(statusfile, pdb_parse_status, NULL);
 
-  *updatefnrest = '\0';
   updateslength= -1;
   cdn = scandir(updatesdir, &cdlist, &ulist_select, alphasort);
   if (cdn == -1) {
@@ -98,17 +98,22 @@ static void cleanupdates(void) {
 
   if (cdn) {
     for (i=0; i<cdn; i++) {
-      strcpy(updatefnrest, cdlist[i]->d_name);
-      parsedb(updatefnbuf, pdb_parse_update, NULL);
+      varbuf_rollback(&updatefn_state);
+      varbuf_add_str(&updatefn, cdlist[i]->d_name);
+      varbuf_end_str(&updatefn);
+      parsedb(updatefn.buf, pdb_parse_update, NULL);
     }
 
     if (cstatus >= msdbrw_write) {
       writedb(statusfile, wdb_must_sync);
 
       for (i=0; i<cdn; i++) {
-        strcpy(updatefnrest, cdlist[i]->d_name);
-        if (unlink(updatefnbuf))
-          ohshite(_("failed to remove incorporated update file %.255s"),updatefnbuf);
+        varbuf_rollback(&updatefn_state);
+        varbuf_add_str(&updatefn, cdlist[i]->d_name);
+        varbuf_end_str(&updatefn);
+        if (unlink(updatefn.buf))
+          ohshite(_("failed to remove incorporated update file %.255s"),
+                  updatefn.buf);
       }
 
       dir_sync_path(updatesdir);
@@ -169,9 +174,10 @@ modstatdb_init(void)
     *fnip->store = dpkg_db_get_path(fnip->suffix);
   }
 
-  updatefnbuf = m_malloc(strlen(updatesdir) + IMPORTANTMAXLEN + 5);
-  strcpy(updatefnbuf, updatesdir);
-  updatefnrest = updatefnbuf + strlen(updatefnbuf);
+  varbuf_init(&updatefn, strlen(updatesdir) + 1 + IMPORTANTMAXLEN);
+  varbuf_add_str(&updatefn, updatesdir);
+  varbuf_end_str(&updatefn);
+  varbuf_snapshot(&updatefn, &updatefn_state);
 
   db_initialized = true;
 }
@@ -188,7 +194,7 @@ modstatdb_done(void)
     free(*fnip->store);
     *fnip->store = NULL;
   }
-  free(updatefnbuf);
+  varbuf_destroy(&updatefn);
 
   db_initialized = false;
 }
@@ -366,15 +372,16 @@ void modstatdb_checkpoint(void) {
   writedb(statusfile, wdb_must_sync);
 
   for (i=0; i<nextupdate; i++) {
-    sprintf(updatefnrest, IMPORTANTFMT, i);
+    varbuf_rollback(&updatefn_state);
+    varbuf_printf(&updatefn, IMPORTANTFMT, i);
 
     /* Have we made a real mess? */
-    if (strlen(updatefnrest) > IMPORTANTMAXLEN)
+    if (varbuf_rollback_len(&updatefn_state) > IMPORTANTMAXLEN)
       internerr("modstatdb update entry name '%s' longer than %d",
-                updatefnrest, IMPORTANTMAXLEN);
+                varbuf_rollback_start(&updatefn_state), IMPORTANTMAXLEN);
 
-    if (unlink(updatefnbuf))
-      ohshite(_("failed to remove my own update file %.255s"),updatefnbuf);
+    if (unlink(updatefn.buf))
+      ohshite(_("failed to remove my own update file %.255s"), updatefn.buf);
   }
 
   dir_sync_path(updatesdir);
@@ -429,17 +436,18 @@ modstatdb_note_core(struct pkginfo *pkg)
   if (fclose(importanttmp))
     ohshite(_("unable to close updated status of '%.250s'"),
             pkg_name(pkg, pnaw_nonambig));
-  sprintf(updatefnrest, IMPORTANTFMT, nextupdate);
-  if (rename(importanttmpfile, updatefnbuf))
+  varbuf_rollback(&updatefn_state);
+  varbuf_printf(&updatefn, IMPORTANTFMT, nextupdate);
+  if (rename(importanttmpfile, updatefn.buf))
     ohshite(_("unable to install updated status of '%.250s'"),
             pkg_name(pkg, pnaw_nonambig));
 
   dir_sync_path(updatesdir);
 
   /* Have we made a real mess? */
-  if (strlen(updatefnrest) > IMPORTANTMAXLEN)
+  if (varbuf_rollback_len(&updatefn_state) > IMPORTANTMAXLEN)
     internerr("modstatdb update entry name '%s' longer than %d",
-              updatefnrest, IMPORTANTMAXLEN);
+              varbuf_rollback_start(&updatefn_state), IMPORTANTMAXLEN);
 
   nextupdate++;