gdb: Improve cache matching criteria for the bfd cache.

Within gdb open bfd objects are reused where possible if an attempt is
made to reopen a file that is already being debugged.  To spot if the on
disc file has changed gdb currently examines the mtime of the file and
compares it to the mtime of the open bfd in the cache.

A problem exists when the on disc file is being rapidly regenerated, as
happens, for example, with automated testing.  In some cases the file is
generated so quickly that the mtime appears not to change, while the on
disc file has changed.

This patch extends the bfd cache to also hold the file size of the file,
the inode of the file, and the device id of the file; gdb can then
compare filename, file size, mtime, inode, and device id to determine if
an existing bfd object can be reused.

gdb/ChangeLog:

	* gdb_bfd.c (struct gdb_bfd_data): Add size, inode, and device id
	field.
	(struct gdb_bfd_cache_search): Likewise.
	(eq_bfd): Compare the size, inode, and device id fields.
	(gdb_bfd_open): Initialise the size, inode, and device id fields.
	(gdb_bfd_ref): Likewise.
	(gdb_bfd_unref): Likewise.

2 files changed, 53 insertions, 1 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 38872b1..8967bc5 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,13 @@
+2015-08-18  Andrew Burgess  <andrew.burgess@embecosm.com>
+
+	* gdb_bfd.c (struct gdb_bfd_data): Add size, inode, and device id
+	field.
+	(struct gdb_bfd_cache_search): Likewise.
+	(eq_bfd): Compare the size, inode, and device id fields.
+	(gdb_bfd_open): Initialise the size, inode, and device id fields.
+	(gdb_bfd_ref): Likewise.
+	(gdb_bfd_unref): Likewise.
+
 2015-08-18  Pedro Alves  <palves@redhat.com>
 
 	* linux-nat.c (linux_nat_always_non_stop_p): If the linux_ops
diff --git a/gdb/gdb_bfd.c b/gdb/gdb_bfd.c
index 1781d80..ee29531 100644
--- a/gdb/gdb_bfd.c
+++ b/gdb/gdb_bfd.c
@@ -69,6 +69,15 @@ struct gdb_bfd_data
   /* The mtime of the BFD at the point the cache entry was made.  */
   time_t mtime;
 
+  /* The file size (in bytes) at the point the cache entry was made.  */
+  off_t size;
+
+  /* The inode of the file at the point the cache entry was made.  */
+  ino_t inode;
+
+  /* The device id of the file at the point the cache entry was made.  */
+  dev_t device_id;
+
   /* This is true if we have determined whether this BFD has any
      sections requiring relocation.  */
   unsigned int relocation_computed : 1;
@@ -112,6 +121,12 @@ struct gdb_bfd_cache_search
   const char *filename;
   /* The mtime.  */
   time_t mtime;
+  /* The file size (in bytes).  */
+  off_t size;
+  /* The inode of the file.  */
+  ino_t inode;
+  /* The device id of the file.  */
+  dev_t device_id;
 };
 
 /* A hash function for BFDs.  */
@@ -136,6 +151,9 @@ eq_bfd (const void *a, const void *b)
   struct gdb_bfd_data *gdata = bfd_usrdata (abfd);
 
   return (gdata->mtime == s->mtime
+	  && gdata->size == s->size
+	  && gdata->inode == s->inode
+	  && gdata->device_id == s->device_id
 	  && strcmp (bfd_get_filename (abfd), s->filename) == 0);
 }
 
@@ -358,9 +376,17 @@ gdb_bfd_open (const char *name, const char *target, int fd)
     {
       /* Weird situation here.  */
       search.mtime = 0;
+      search.size = 0;
+      search.inode = 0;
+      search.device_id = 0;
     }
   else
-    search.mtime = st.st_mtime;
+    {
+      search.mtime = st.st_mtime;
+      search.size = st.st_size;
+      search.inode = st.st_ino;
+      search.device_id = st.st_dev;
+    }
 
   /* Note that this must compute the same result as hash_bfd.  */
   hash = htab_hash_string (name);
@@ -435,6 +461,7 @@ gdb_bfd_close_or_warn (struct bfd *abfd)
 void
 gdb_bfd_ref (struct bfd *abfd)
 {
+  struct stat buf;
   struct gdb_bfd_data *gdata;
   void **slot;
 
@@ -455,7 +482,19 @@ gdb_bfd_ref (struct bfd *abfd)
   gdata = bfd_zalloc (abfd, sizeof (struct gdb_bfd_data));
   gdata->refc = 1;
   gdata->mtime = bfd_get_mtime (abfd);
+  gdata->size = bfd_get_size (abfd);
   gdata->archive_bfd = NULL;
+  if (bfd_stat (abfd, &buf) == 0)
+    {
+      gdata->inode = buf.st_ino;
+      gdata->device_id = buf.st_dev;
+    }
+  else
+    {
+      /* The stat failed.  */
+      gdata->inode = 0;
+      gdata->device_id = 0;
+    }
   bfd_usrdata (abfd) = gdata;
 
   bfd_alloc_data (abfd);
@@ -495,6 +534,9 @@ gdb_bfd_unref (struct bfd *abfd)
       void **slot;
 
       search.mtime = gdata->mtime;
+      search.size = gdata->size;
+      search.inode = gdata->inode;
+      search.device_id = gdata->device_id;
       slot = htab_find_slot_with_hash (gdb_bfd_cache, &search, hash,
 				       NO_INSERT);