Further comment improvements in wal.c. No code changes.

FossilOrigin-Name: 346388007de585083dc67ad865b91db7c7d7b78c10a06f8bb7c48767c326c47e

diff --git a/manifest b/manifest
index c2392bfdac46c81b18a3fc6fe7f6db461405953b..e067bdd554094a03c3386fa3331c7eb8a273a87b 100644 (file)
--- a/manifest
+++ b/manifest
@@ -1,5 +1,5 @@
-C Improved\scomments\sand\svariable\snames\sin\sthe\sread-only\sWAL\slogic.
-D 2017-11-10T20:00:50.239
+C Further\scomment\simprovements\sin\swal.c.\s\sNo\scode\schanges.
+D 2017-11-11T13:30:44.595
 F Makefile.in 5bae3f2f3d42f2ad52b141562d74872c97ac0fca6c54953c91bb150a0e6427a8
 F Makefile.linux-gcc 7bc79876b875010e8c8f9502eb935ca92aa3c434
 F Makefile.msc 3a5cb477ec3ce5274663b693164e349db63348667cd45bad78cc13d580b691e2
@@ -543,7 +543,7 @@ F src/vdbesort.c 731a09e5cb9e96b70c394c1b7cf3860fbe84acca7682e178615eb941a3a0ef2
 F src/vdbetrace.c 48e11ebe040c6b41d146abed2602e3d00d621d7ebe4eb29b0a0f1617fd3c2f6c
 F src/vtab.c 0e4885495172e1bdf54b12cce23b395ac74ef5729031f15e1bc1e3e6b360ed1a
 F src/vxworks.h d2988f4e5a61a4dfe82c6524dd3d6e4f2ce3cdb9
-F src/wal.c 47f8a4493e077b312399b62fd36fd4e8b70535dcd8d9a5caf18b8be019fcf420
+F src/wal.c 0e19d4fbb52085697509f9407176e732d440f372f49ca08510a65d1aa976bd07
 F src/wal.h 8de5d2d3de0956d6f6cb48c83a4012d5f227b8fe940f3a349a4b7e85ebcb492a
 F src/walker.c d591e8a9ccf60abb010966b354fcea4aa08eba4d83675c2b281a8764c76cc22f
 F src/where.c b7a075f5fb3d912a891dcc3257f538372bb4a1622dd8ca7d752ad95ce8949ba4
@@ -1669,7 +1669,7 @@ F vsixtest/vsixtest.tcl 6a9a6ab600c25a91a7acc6293828957a386a8a93
 F vsixtest/vsixtest.vcxproj.data 2ed517e100c66dc455b492e1a33350c1b20fbcdc
 F vsixtest/vsixtest.vcxproj.filters 37e51ffedcdb064aad6ff33b6148725226cd608e
 F vsixtest/vsixtest_TemporaryKey.pfx e5b1b036facdb453873e7084e1cae9102ccc67a0
-P 5a384be6979b783d1f3af2ac6307e7e731c415d052f9405f04c0216f59414633
-R 4efdec1d3561b29c7744e3cfcf4d5f7b
+P d3c25740eec9a2a41c29e6e488fcf6587c1fb821147a442c29439b25a92154a5
+R cc050d490699d35569ec56a8515bac08
 U drh
-Z df395ed2f6beb277e98c44e2de48326a
+Z 1e870a391397f9beedb8e100937bac44

diff --git a/manifest.uuid b/manifest.uuid
index 354097e8fa011b0e8751aa6eab7b60a9d59ee091..3a4ec0326576d9fc59f186a191107a78082fcc12 100644 (file)
--- a/manifest.uuid
+++ b/manifest.uuid
@@ -1 +1 @@
-d3c25740eec9a2a41c29e6e488fcf6587c1fb821147a442c29439b25a92154a5
\ No newline at end of file
+346388007de585083dc67ad865b91db7c7d7b78c10a06f8bb7c48767c326c47e
\ No newline at end of file

diff --git a/src/wal.c b/src/wal.c
index aff0e97fa2ef41e7f3158defd39efce70c8ab44b..87fbda12d682e13d8cf25751b68ee566d55f099d 100644 (file)
--- a/src/wal.c
+++ b/src/wal.c
@@ -2230,16 +2230,23 @@ static int walBeginShmUnreliable(Wal *pWal, int *pChanged){
   ** the xShmMap() routine of the VFS and looking to see if the return
   ** is SQLITE_READONLY instead of SQLITE_READONLY_CANTINIT.
   **
-  ** Once sqlite3OsShmMap() has been called for a file and has returned
-  ** any SQLITE_READONLY value, it must SQLITE_READONLY or
-  ** SQLITE_READONLY_CANTINIT or some error for all subsequent invocations,
-  ** until sqlite3OsShmUnmap() has been called.  This is a requirement
-  ** on the VFS implementation.
-  **
   ** If the shared-memory is now "reliable" return WAL_RETRY, which will
   ** cause the heap-memory WAL-index to be discarded and the actual
   ** shared memory to be used in its place.
-  */
+  **
+  ** This step is important because, even though this connection is holding
+  ** the WAL_READ_LOCK(0) which prevents a checkpoint, a writer might
+  ** have already checkpointed the WAL file and, while the current
+  ** is active, wrap the WAL and start overwriting frames that this
+  ** process wants to use.
+  **
+  ** Once sqlite3OsShmMap() has been called for an sqlite3_file and has
+  ** returned any SQLITE_READONLY value, it must return only SQLITE_READONLY
+  ** or SQLITE_READONLY_CANTINIT or some error for all subsequent invocations,
+  ** even if some external agent does a "chmod" to make the shared-memory
+  ** writable by us, until sqlite3OsShmUnmap() has been called.
+  ** This is a requirement on the VFS implementation.
+   */
   rc = sqlite3OsShmMap(pWal->pDbFd, 0, WALINDEX_PGSZ, 0, &pDummy);
   assert( rc!=SQLITE_OK ); /* SQLITE_OK not possible for read-only connection */
   if( rc!=SQLITE_READONLY_CANTINIT ){
@@ -2247,19 +2254,14 @@ static int walBeginShmUnreliable(Wal *pWal, int *pChanged){
     goto begin_unreliable_shm_out;
   }
 
-  /* Reach this point only if the real shared-memory is still unreliable.
+  /* We reach this point only if the real shared-memory is still unreliable.
   ** Assume the in-memory WAL-index substitute is correct and load it
   ** into pWal->hdr.
   */
   memcpy(&pWal->hdr, (void*)walIndexHdr(pWal), sizeof(WalIndexHdr));
 
-  /* The WAL_READ_LOCK(0) lock held by this client prevents a checkpoint
-  ** from taking place. But it does not prevent the wal from being wrapped
-  ** if a checkpoint has already taken place. This means that if another
-  ** client is connected at this point, it may have already checkpointed 
-  ** the entire wal. In that case it would not be safe to continue with
-  ** the this transaction, as the other client may overwrite wal 
-  ** frames that this client is still using.
+  /* Make sure some writer hasn't come in and changed the WAL file out
+  ** from under us, then disconnected, while we were not looking.
   */
   rc = sqlite3OsFileSize(pWal->pWalFd, &szWal);
   if( rc!=SQLITE_OK ){
@@ -2283,6 +2285,9 @@ static int walBeginShmUnreliable(Wal *pWal, int *pChanged){
     goto begin_unreliable_shm_out;
   }
   if( memcmp(&pWal->hdr.aSalt, &aBuf[16], 8) ){
+    /* Some writer has wrapped the WAL file while we were not looking.
+    ** Return WAL_RETRY which will cause the in-memory WAL-index to be
+    ** rebuilt. */
     rc = WAL_RETRY;
     goto begin_unreliable_shm_out;
   }