master 4b6b9a7acdc 2/4: Avoid some GC when locking/unlocking files

[Paul Eggert]

branch: master
commit 4b6b9a7acdc4f7d0594caaaa382e2e633f8f1225
Author: Paul Eggert <eggert@cs.ucla.edu>
Commit: Paul Eggert <eggert@cs.ucla.edu>

    Avoid some GC when locking/unlocking files
    
    * src/filelock.c (lock_file_1, current_lock_owner):
    Don’t possibly invoke the garbage collector when
    comparing lock file contents to host names.
---
 src/filelock.c | 62 +++++++++++++++++++++++++++++++++++-----------------------
 1 file changed, 37 insertions(+), 25 deletions(-)

diff --git a/src/filelock.c b/src/filelock.c
index 55ab15feb8d..cdf9e6f0ffc 100644
--- a/src/filelock.c
+++ b/src/filelock.c
@@ -271,27 +271,29 @@ lock_file_1 (Lisp_Object lfname, bool force)
   intmax_t boot = get_boot_sec ();
   Lisp_Object luser_name = Fuser_login_name (Qnil);
   Lisp_Object lhost_name = Fsystem_name ();
-
-  /* Protect against the extremely unlikely case of the host name
-     containing an @ character.  */
-  if (!NILP (lhost_name) && strchr (SSDATA (lhost_name), '@'))
-    lhost_name = CALLN (Ffuncall, Qstring_replace,
-                       build_string ("@"), build_string ("-"),
-                       lhost_name);
-
   char const *user_name = STRINGP (luser_name) ? SSDATA (luser_name) : "";
   char const *host_name = STRINGP (lhost_name) ? SSDATA (lhost_name) : "";
   char lock_info_str[MAX_LFINFO + 1];
   intmax_t pid = getpid ();
 
-  char const *lock_info_fmt = (boot
-                              ? "%s@%s.%"PRIdMAX":%"PRIdMAX
-                              : "%s@%s.%"PRIdMAX);
-  int len = snprintf (lock_info_str, sizeof lock_info_str,
-                     lock_info_fmt, user_name, host_name, pid, boot);
+  int room = sizeof lock_info_str;
+  int len = snprintf (lock_info_str, room, "%s@", user_name);
   if (! (0 <= len && len < sizeof lock_info_str))
     return ENAMETOOLONG;
-
+  /* Protect against the extremely unlikely case of the host name
+     containing an @ character.  */
+  for (; *host_name; len++, host_name++)
+    {
+      if (! (len < sizeof lock_info_str - 1))
+       return ENAMETOOLONG;
+      lock_info_str[len] = *host_name == '@' ? '-' : *host_name;
+    }
+  char const *lock_info_fmt = boot ? ".%"PRIdMAX":%"PRIdMAX : ".%"PRIdMAX;
+  room = sizeof lock_info_str - len;
+  int suffixlen = snprintf (lock_info_str + len, room,
+                           lock_info_fmt, pid, boot);
+  if (! (0 <= suffixlen && suffixlen < room))
+    return ENAMETOOLONG;
   return create_lock_file (SSDATA (lfname), lock_info_str, force);
 }
 
@@ -448,22 +450,32 @@ current_lock_owner (lock_info_type *owner, Lisp_Object 
lfname)
   if (lfinfo_end != owner->user + lfinfolen)
     return EINVAL;
 
+  char *linkhost = at + 1;
+  ptrdiff_t linkhostlen = dot - linkhost;
   Lisp_Object system_name = Fsystem_name ();
   /* If `system-name' returns nil, that means we're in a
      --no-build-details Emacs, and the name part of the link (e.g.,
      .#test.txt -> larsi@.118961:1646577954) is an empty string.  */
+  bool on_current_host;
   if (NILP (system_name))
-    system_name = build_string ("");
-  /* Protect against the extremely unlikely case of the host name
-     containing an @ character.  */
-  else if (strchr (SSDATA (system_name), '@'))
-    system_name = CALLN (Ffuncall, intern ("string-replace"),
-                        build_string ("@"), build_string ("-"),
-                        system_name);
-  /* On current host?  */
-  if (STRINGP (system_name)
-      && dot - (at + 1) == SBYTES (system_name)
-      && memcmp (at + 1, SSDATA (system_name), SBYTES (system_name)) == 0)
+    on_current_host = linkhostlen == 0;
+  else
+    {
+      on_current_host = linkhostlen == SBYTES (system_name);
+      if (on_current_host)
+       {
+         /* Protect against the extremely unlikely case of the host
+            name containing '@'.  */
+         char *sysname = SSDATA (system_name);
+         for (ptrdiff_t i = 0; i < linkhostlen; i++)
+           if (linkhost[i] != (sysname[i] == '@' ? '-' : sysname[i]))
+             {
+               on_current_host = false;
+               break;
+             }
+       }
+    }
+  if (on_current_host)
     {
       if (pid == getpid ())
         return I_OWN_IT;