Lock the wal file for all snapshot transactions, even if they would not

otherwise require this, preventing checkpointers and writers from wrapping the
wal file. This means that if one connection has an open snapshot transaction
it is guaranteed that a second connection can open a transaction on the same
snapshot.

FossilOrigin-Name: b81a31495bd27c1d96f7df653da3502054240cb5acf66b860da7f0f9b422a524

manifest

@@ -1,5 +1,5 @@
-C Add\sexperimental\sfeature\sto\sdetect\sthreading\sbugs\sin\sapps\sthat\suse\nSQLITE_CONFIG_MULTITHREADED.\sEnabled\sat\scompile\stime\susing\nSQLITE_ENABLE_MULTITHREADED_CHECKS.
-D 2017-11-28T07:52:00.416
+C Lock\sthe\swal\sfile\sfor\sall\ssnapshot\stransactions,\seven\sif\sthey\swould\snot\notherwise\srequire\sthis,\spreventing\scheckpointers\sand\swriters\sfrom\swrapping\sthe\nwal\sfile.\sThis\smeans\sthat\sif\sone\sconnection\shas\san\sopen\ssnapshot\stransaction\nit\sis\sguaranteed\sthat\sa\ssecond\sconnection\scan\sopen\sa\stransaction\son\sthe\ssame\nsnapshot.
+D 2017-11-28T13:39:41.135
 F Makefile.in 6a879cbf01e37f9eac131414955f71774b566502d9a57ded1b8585b507503cb8
 F Makefile.linux-gcc 7bc79876b875010e8c8f9502eb935ca92aa3c434
 F Makefile.msc e5d7606238f55816da99f719969598df5b091aa2e9a6935c9412fcae8f53fc44
@@ -551,7 +551,7 @@ F src/vdbesort.c 731a09e5cb9e96b70c394c1b7cf3860fbe84acca7682e178615eb941a3a0ef2
 F src/vdbetrace.c 48e11ebe040c6b41d146abed2602e3d00d621d7ebe4eb29b0a0f1617fd3c2f6c
 F src/vtab.c 0e4885495172e1bdf54b12cce23b395ac74ef5729031f15e1bc1e3e6b360ed1a
 F src/vxworks.h d2988f4e5a61a4dfe82c6524dd3d6e4f2ce3cdb9
-F src/wal.c beeb71e4eab65dbf0d95f2717efc6ca3c0f5b3090ce67f3de63828f39a6ff053
+F src/wal.c 5a3f464edd64596f601683ed321d12e6fd93c5fb9afdfb3653d6ffd0fee9c48f
 F src/wal.h 8de5d2d3de0956d6f6cb48c83a4012d5f227b8fe940f3a349a4b7e85ebcb492a
 F src/walker.c da987a20d40145c0a03c07d8fefcb2ed363becc7680d0500d9c79915591f5b1f
 F src/where.c 9752b68e03e2044f0faa4708fabb0189769067b660bffa931e1fd65736269659
@@ -1223,6 +1223,7 @@ F test/skipscan5.test 67817a4b6857c47e0e33ba3e506da6f23ef68de2
 F test/skipscan6.test 5866039d03a56f5bd0b3d172a012074a1d90a15b
 F test/snapshot.test 85735bd997a4f6d710140c28fd860519a299649f
 F test/snapshot2.test 925e42427e923262db63c9d7155183f889e3e99feaedec4075f659e51608344f
+F test/snapshot3.test 9719443594a04778861bd20d12596c5f880af177d6cd62f111da3198cafc6096
 F test/snapshot_fault.test 52c5e97ebd218846a8ae2da4d147d3e77d71f963
 F test/soak.test 0b5b6375c9f4110c828070b826b3b4b0bb65cd5f
 F test/softheap1.test 843cd84db9891b2d01b9ab64cef3e9020f98d087
@@ -1678,8 +1679,8 @@ F vsixtest/vsixtest.tcl 6a9a6ab600c25a91a7acc6293828957a386a8a93
 F vsixtest/vsixtest.vcxproj.data 2ed517e100c66dc455b492e1a33350c1b20fbcdc
 F vsixtest/vsixtest.vcxproj.filters 37e51ffedcdb064aad6ff33b6148725226cd608e
 F vsixtest/vsixtest_TemporaryKey.pfx e5b1b036facdb453873e7084e1cae9102ccc67a0
-P e6b89304695be371978e65dddd710c8bd563c66b9c94d23165142b6c235c82e1 12a23c0a66fac5c9674120b390f6abaeaba3f7ff04693b281af1eefb93d6f47c
-R dd8aff31239aebcaa5e5379a4673912d
-T +closed 12a23c0a66fac5c9674120b390f6abaeaba3f7ff04693b281af1eefb93d6f47c
+P 40b598c8392f030f6ed8c63ce81cb0426bb3984397c19c756215f6a569a40164 d71eeaab9ecdeed772047498b781be1f0be0655af284b94cf676bb408ceea8b1
+R e37ca38ceb47340e2d23476ad044f736
+T +closed d71eeaab9ecdeed772047498b781be1f0be0655af284b94cf676bb408ceea8b1
 U dan
-Z bdded6a608898929c58df7d45769809f
+Z a9d291c094a58941f666434dce6709fb

manifest.uuid

@@ -1 +1 @@
-40b598c8392f030f6ed8c63ce81cb0426bb3984397c19c756215f6a569a40164
+b81a31495bd27c1d96f7df653da3502054240cb5acf66b860da7f0f9b422a524

src/wal.c

@@ -2494,8 +2494,7 @@ static int walTryBeginRead(Wal *pWal, int *pChanged, int useWal, int cnt){
   pInfo = walCkptInfo(pWal);
   if( !useWal && pInfo->nBackfill==pWal->hdr.mxFrame
 #ifdef SQLITE_ENABLE_SNAPSHOT
-   && (pWal->pSnapshot==0 || pWal->hdr.mxFrame==0
-     || 0==memcmp(&pWal->hdr, pWal->pSnapshot, sizeof(WalIndexHdr)))
+   && (pWal->pSnapshot==0 || pWal->hdr.mxFrame==0)
 #endif
   ){
     /* The WAL has been completely backfilled (or it is empty).

test/snapshot3.test

@@ -0,0 +1,100 @@
+# 2016 September 23
+#
+# The author disclaims copyright to this source code.  In place of
+# a legal notice, here is a blessing:
+#
+#    May you do good and not evil.
+#    May you find forgiveness for yourself and forgive others.
+#    May you share freely, never taking more than you give.
+#
+#***********************************************************************
+# This file implements regression tests for SQLite library. The focus
+# of this file is the sqlite3_snapshot_xxx() APIs.
+#
+
+set testdir [file dirname $argv0]
+source $testdir/tester.tcl
+ifcapable !snapshot {finish_test; return}
+set testprefix snapshot3
+
+# This test does not work with the inmemory_journal permutation. The reason
+# is that each connection opened as part of this permutation executes
+# "PRAGMA journal_mode=memory", which fails if the database is in wal mode
+# and there are one or more existing connections.
+if {[permutation]=="inmemory_journal"} {
+  finish_test
+  return
+}
+
+#-------------------------------------------------------------------------
+# This block of tests verifies that it is not possible to wrap the wal
+# file - using a writer or a "PRAGMA wal_checkpoint = TRUNCATE" - while
+# there is an open snapshot transaction (transaction opened using
+# sqlite3_snapshot_open()).
+#
+do_execsql_test 1.0 {
+  CREATE TABLE t1(y);
+  PRAGMA journal_mode = wal;
+  INSERT INTO t1 VALUES(1);
+  INSERT INTO t1 VALUES(2);
+  INSERT INTO t1 VALUES(3);
+  INSERT INTO t1 VALUES(4);
+} {wal}
+
+do_test 1.1 {
+  sqlite3 db2 test.db
+  sqlite3 db3 test.db
+
+  execsql {SELECT * FROM sqlite_master} db2
+  execsql {SELECT * FROM sqlite_master} db3
+
+  db2 trans { set snap [sqlite3_snapshot_get_blob db2 main] }
+  db2 eval { SELECT * FROM t1 }
+} {1 2 3 4}
+
+do_test 1.2 {
+  execsql BEGIN db2
+  sqlite3_snapshot_open_blob db2 main $snap
+  db2 eval { SELECT * FROM t1 }
+} {1 2 3 4}
+
+do_test 1.2 {
+  execsql END db2
+  execsql { PRAGMA wal_checkpoint }
+
+  execsql BEGIN db2
+  sqlite3_snapshot_open_blob db2 main $snap
+  db2 eval { SELECT * FROM t1 }
+} {1 2 3 4}
+
+set sz [file size test.db-wal]
+do_test 1.3 {
+  execsql { PRAGMA wal_checkpoint = truncate }
+  file size test.db-wal
+} $sz
+
+do_test 1.4 {
+  execsql BEGIN db3
+  list [catch { sqlite3_snapshot_open_blob db3 main $snap } msg] $msg
+} {0 {}}
+
+do_test 1.5 {
+  db3 eval { SELECT * FROM t1; END }
+} {1 2 3 4}
+
+do_test 1.6 {
+  db2 eval { SELECT * FROM t1; END }
+} {1 2 3 4}
+
+do_test 1.7 {
+  execsql { PRAGMA wal_checkpoint = truncate }
+  file size test.db-wal
+} 0
+
+do_test 1.8 {
+  execsql BEGIN db3
+  list [catch { sqlite3_snapshot_open_blob db3 main $snap } msg] $msg
+} {1 SQLITE_BUSY_SNAPSHOT}
+
+finish_test
+