Add tracing logic to the shared-cache locks in btree.c. The tracing is

off by default.  Enable by changing a single "#if 0" into "#if 1" and
recompiling.  Debugging code only - no changes to release builds.

FossilOrigin-Name: f2b943f97ad7e47848ac6df3a3a1eba134b9e63c4a631f8eaf8bda77cc02ba7b

manifest

@@ -1,5 +1,5 @@
-C Add\sthe\stest_oom_breakpoint()\sroutine\son\sdebug\sbuilds,\sto\sserve\sas\sa\nconvenient\sbreakpoint\sto\sintercept\sOOM\sconditions.
-D 2024-02-01T11:38:58.054
+C Add\stracing\slogic\sto\sthe\sshared-cache\slocks\sin\sbtree.c.\s\sThe\stracing\sis\noff\sby\sdefault.\s\sEnable\sby\schanging\sa\ssingle\s"#if\s0"\sinto\s"#if\s1"\sand\nrecompiling.\s\sDebugging\scode\sonly\s-\sno\schanges\sto\srelease\sbuilds.
+D 2024-02-01T14:17:01.624
 F .fossil-settings/empty-dirs dbb81e8fc0401ac46a1491ab34a7f2c7c0452f2f06b54ebb845d024ca8283ef1
 F .fossil-settings/ignore-glob 35175cdfcf539b2318cb04a9901442804be81cd677d8b889fcc9149c21f239ea
 F LICENSE.md df5091916dbb40e6e9686186587125e1b2ff51f022cc334e886c19a0e9982724
@@ -677,7 +677,7 @@ F src/auth.c 19b7ccacae3dfba23fc6f1d0af68134fa216e9040e53b0681b4715445ea030b4
 F src/backup.c 5c97e8023aab1ce14a42387eb3ae00ba5a0644569e3476f38661fa6f824c3523
 F src/bitvec.c 9eac5f42c11914d5ef00a75605bb205e934f435c579687f985f1f8b0995c8645
 F src/btmutex.c 79a43670447eacc651519a429f6ece9fd638563cf95b469d6891185ddae2b522
-F src/btree.c c64df2b1623501e397128261de58d3ab44c301e4eb993a4055aa971444420200
+F src/btree.c 186359bd695150d22b9f385ffb487b9a029ecea158658eed3a0268ee63ecbf9c
 F src/btree.h 03e3356f5208bcab8eed4e094240fdac4a7f9f5ddf5e91045ce589f67d47c240
 F src/btreeInt.h 3e2589726c4f105e653461814f65857465da68be1fac688de340c43b873f4062
 F src/build.c 05c9eb387638982092988d2ef4c00a91b0c29baa1b85923f082ce0cf2903ea7e
@@ -2161,8 +2161,8 @@ F vsixtest/vsixtest.tcl 6a9a6ab600c25a91a7acc6293828957a386a8a93
 F vsixtest/vsixtest.vcxproj.data 2ed517e100c66dc455b492e1a33350c1b20fbcdc
 F vsixtest/vsixtest.vcxproj.filters 37e51ffedcdb064aad6ff33b6148725226cd608e
 F vsixtest/vsixtest_TemporaryKey.pfx e5b1b036facdb453873e7084e1cae9102ccc67a0
-P 44b5524d522e749ad6bf76c94d754ff16c309c32439ec46802924663f64e8b09
-R cf60142a32287adf1c10b35b8571b6b1
+P e45df7dcd6b5766d7593ee87e59dd422a217cce0a1a8d369c03144bb21859428
+R 1c4d155f92c2e065cfacf61f3e9f41bd
 U drh
-Z 5e6025aea61024ad1af737cfb95ff572
+Z 977fd1472c72116436cf99418d5cf571
 # Remove this line to create a well-formed Fossil manifest.

manifest.uuid

@@ -1 +1 @@
-e45df7dcd6b5766d7593ee87e59dd422a217cce0a1a8d369c03144bb21859428
+f2b943f97ad7e47848ac6df3a3a1eba134b9e63c4a631f8eaf8bda77cc02ba7b

src/btree.c

@@ -151,8 +151,47 @@ int corruptPageError(int lineno, MemPage *p){
 # define SQLITE_CORRUPT_PAGE(pMemPage) SQLITE_CORRUPT_PGNO(pMemPage->pgno)
 #endif
 
+/* Default value for SHARED_LOCK_TRACE macro if shared-cache is disabled
+** or if the lock tracking is disabled.  This is always the value for
+** release builds.
+*/
+#define SHARED_LOCK_TRACE(X,MSG,TAB,TYPE)  /*no-op*/
+
 #ifndef SQLITE_OMIT_SHARED_CACHE
 
+#if 0
+/*  ^----  Change to 1 and recompile to enable shared-lock tracing
+**         for debugging purposes.
+**
+** Print all shared-cache locks on a BtShared.  Debugging use only.
+*/
+static void sharedLockTrace(
+  BtShared *pBt,
+  const char *zMsg,
+  int iRoot,
+  int eLockType
+){
+  BtLock *pLock;
+  if( iRoot>0 ){
+    printf("%s-%p %u%s:", zMsg, pBt, iRoot, eLockType==READ_LOCK?"R":"W");
+  }else{
+    printf("%s-%p:", zMsg, pBt);
+  }
+  for(pLock=pBt->pLock; pLock; pLock=pLock->pNext){
+    printf(" %p/%u%s", pLock->pBtree, pLock->iTable,
+           pLock->eLock==READ_LOCK ? "R" : "W");
+    while( pLock->pNext && pLock->pBtree==pLock->pNext->pBtree ){
+      pLock = pLock->pNext;
+      printf(",%u%s", pLock->iTable, pLock->eLock==READ_LOCK ? "R" : "W");
+    }
+  }
+  printf("\n");
+  fflush(stdout);
+}
+#undef SHARED_LOCK_TRACE
+#define SHARED_LOCK_TRACE(X,MSG,TAB,TYPE)  sharedLockTrace(X,MSG,TAB,TYPE)
+#endif /* Shared-lock tracing */
+
 #ifdef SQLITE_DEBUG
 /*
 **** This function is only used as part of an assert() statement. ***
@@ -229,6 +268,8 @@ static int hasSharedCacheTableLock(
     iTab = iRoot;
   }
 
+  SHARED_LOCK_TRACE(pBtree->pBt,"hasLock",iRoot,eLockType);
+
   /* Search for the required lock. Either a write-lock on root-page iTab, a
   ** write-lock on the schema table, or (if the client is reading) a
   ** read-lock on iTab will suffice. Return 1 if any of these are found.  */
@@ -362,6 +403,8 @@ static int setSharedCacheTableLock(Btree *p, Pgno iTable, u8 eLock){
   BtLock *pLock = 0;
   BtLock *pIter;
 
+  SHARED_LOCK_TRACE(pBt,"setLock", iTable, eLock);
+
   assert( sqlite3BtreeHoldsMutex(p) );
   assert( eLock==READ_LOCK || eLock==WRITE_LOCK );
   assert( p->db!=0 );
@@ -429,6 +472,8 @@ static void clearAllSharedCacheTableLocks(Btree *p){
   assert( p->sharable || 0==*ppIter );
   assert( p->inTrans>0 );
 
+  SHARED_LOCK_TRACE(pBt, "clearAllLocks", 0, 0);
+
   while( *ppIter ){
     BtLock *pLock = *ppIter;
     assert( (pBt->btsFlags & BTS_EXCLUSIVE)==0 || pBt->pWriter==pLock->pBtree );
@@ -467,6 +512,9 @@ static void clearAllSharedCacheTableLocks(Btree *p){
 */
 static void downgradeAllSharedCacheTableLocks(Btree *p){
   BtShared *pBt = p->pBt;
+
+  SHARED_LOCK_TRACE(pBt, "downgradeLocks", 0, 0);
+
   if( pBt->pWriter==p ){
     BtLock *pLock;
     pBt->pWriter = 0;