Create the new RCStr class of strings and try to use them for JSON storage.

FossilOrigin-Name: c1b8725089bb3d006ec69add28f4fcb3f4e79412c7f438b5b1067c2227e77b9c

4 files changed, 245 insertions, 15 deletions

diff --git a/src/json.c b/src/json.c
index 0ce68f1fc..3bfa0c748 100644
--- a/src/json.c
+++ b/src/json.c
@@ -191,7 +191,7 @@ static void jsonInit(JsonString *p, sqlite3_context *pCtx){
 ** initial state.
 */
 static void jsonReset(JsonString *p){
-  if( !p->bStatic ) sqlite3_free(p->zBuf);
+  if( !p->bStatic ) sqlite3RCStrUnref(p->zBuf);
   jsonZero(p);
 }
 
@@ -212,7 +212,7 @@ static int jsonGrow(JsonString *p, u32 N){
   char *zNew;
   if( p->bStatic ){
     if( p->bErr ) return 1;
-    zNew = sqlite3_malloc64(nTotal);
+    zNew = sqlite3RCStrNew(nTotal);
     if( zNew==0 ){
       jsonOom(p);
       return SQLITE_NOMEM;
@@ -221,12 +221,12 @@ static int jsonGrow(JsonString *p, u32 N){
     p->zBuf = zNew;
     p->bStatic = 0;
   }else{
-    zNew = sqlite3_realloc64(p->zBuf, nTotal);
-    if( zNew==0 ){
-      jsonOom(p);
+    p->zBuf = sqlite3RCStrResize(p->zBuf, nTotal);
+    if( p->zBuf==0 ){
+      p->bErr = 1;
+      jsonZero(p);
       return SQLITE_NOMEM;
     }
-    p->zBuf = zNew;
   }
   p->nAlloc = nTotal;
   return SQLITE_OK;
@@ -514,16 +514,26 @@ static void jsonAppendValue(
 
 
 /* Make the JSON in p the result of the SQL function.
+**
+** The JSON string is reset.
 */
 static void jsonResult(JsonString *p){
   if( p->bErr==0 ){
-    jsonAppendChar(p, 0);
-    sqlite3_result_text64(p->pCtx, p->zBuf, p->nUsed-1,
-                          p->bStatic ? SQLITE_TRANSIENT : sqlite3_free,
-                          SQLITE_UTF8);
-    jsonZero(p);
+    if( p->bStatic ){
+      sqlite3_result_text64(p->pCtx, p->zBuf, p->nUsed, SQLITE_TRANSIENT,
+                            SQLITE_UTF8);
+    }else{
+      jsonAppendChar(p, 0);
+      p->nUsed--;
+      sqlite3RCStrRef(p->zBuf);
+      sqlite3_result_text64(p->pCtx, p->zBuf, p->nUsed,
+                            (void(*)(void*))sqlite3RCStrUnref,
+                            SQLITE_UTF8);
+    }
+  }else{
+    sqlite3_result_error_nomem(p->pCtx);
   }
-  assert( p->bStatic );
+  jsonReset(p);
 }
 
 /**************************************************************************
@@ -1885,7 +1895,7 @@ static JsonNode *jsonLookupStep(
   JsonNode *pRoot = &pParse->aNode[iRoot];
   while( (pRoot->jnFlags & JNODE_REPLACE)!=0 ){
     u32 idx = (u32)(pRoot - pParse->aNode);
-    u32 i = pParse->iSubst;
+    i = pParse->iSubst;
     while( 1 /*exit-by-break*/ ){
       assert( i<pParse->nNode );
       assert( pParse->aNode[i].eType==JSON_SUBST );
@@ -2950,7 +2960,8 @@ static void jsonArrayCompute(sqlite3_context *ctx, int isFinal){
       assert( pStr->bStatic );
     }else if( isFinal ){
       sqlite3_result_text(ctx, pStr->zBuf, (int)pStr->nUsed,
-                          pStr->bStatic ? SQLITE_TRANSIENT : sqlite3_free);
+                          pStr->bStatic ? SQLITE_TRANSIENT :
+                              (void(*)(void*))sqlite3RCStrUnref);
       pStr->bStatic = 1;
     }else{
       sqlite3_result_text(ctx, pStr->zBuf, (int)pStr->nUsed, SQLITE_TRANSIENT);
@@ -3058,7 +3069,8 @@ static void jsonObjectCompute(sqlite3_context *ctx, int isFinal){
       assert( pStr->bStatic );
     }else if( isFinal ){
       sqlite3_result_text(ctx, pStr->zBuf, (int)pStr->nUsed,
-                          pStr->bStatic ? SQLITE_TRANSIENT : sqlite3_free);
+                          pStr->bStatic ? SQLITE_TRANSIENT :
+                          (void(*)(void*))sqlite3RCStrUnref);
       pStr->bStatic = 1;
     }else{
       sqlite3_result_text(ctx, pStr->zBuf, (int)pStr->nUsed, SQLITE_TRANSIENT);
diff --git a/src/printf.c b/src/printf.c
index 3fb1a322a..7b8428816 100644
--- a/src/printf.c
+++ b/src/printf.c
@@ -1366,3 +1366,146 @@ void sqlite3_str_appendf(StrAccum *p, const char *zFormat, ...){
   sqlite3_str_vappendf(p, zFormat, ap);
   va_end(ap);
 }
+
+
+/*****************************************************************************
+** Reference counted string storage
+*****************************************************************************/
+
+/*
+** Increase the reference count of the string by one.
+**
+** The input parameter is returned.
+*/
+char *sqlite3RCStrRef(char *z){
+  RCStr *p = (RCStr*)z;
+  assert( p!=0 );
+  p--;
+  p->nRCRef++;
+  return z;
+}
+
+/*
+** Decrease the reference count by one.  Free the string when the
+** reference count reaches zero.
+*/
+void sqlite3RCStrUnref(char *z){
+  RCStr *p = (RCStr*)z;
+  assert( p!=0 );
+  p--;
+  assert( p->nRCRef>0 );
+  assert( p->uMagic==SQLITE_RCSTR_MAGIC );
+  if( p->nRCRef>=2 ){
+    p->nRCRef--;
+  }else{
+    if( p->xFree ) p->xFree(p->pAttach);
+#ifdef SQLITE_DEBUG
+    p->uMagic = 0;
+#endif
+    sqlite3_free(p);
+  }
+}
+
+/*
+** Return true if the reference count on the string is exactly one, meaning
+** that the string can be modified.  Return false if the reference count
+** is greater than one.
+*/
+int sqlite3RCStrIsWriteable(char *z){
+  RCStr *p = (RCStr*)z;
+  assert( p!=0 );
+  p--;
+  assert( p->nRCRef>0 );
+  assert( p->uMagic==SQLITE_RCSTR_MAGIC );
+  return p->nRCRef==1;
+}
+
+/*
+** Create a new string that is capable of holding N bytes of text, not counting
+** the zero byte at the end.  The string is uninitialized.
+**
+** The reference count is initially 1.  Call sqlite3RCStrUnref() to free the
+** newly allocated string.
+**
+** This routine returns 0 on an OOM.
+*/
+char *sqlite3RCStrNew(u64 N){
+  RCStr *p = sqlite3_malloc64( N + sizeof(*p) );
+  if( p==0 ) return 0;
+  p->nRCRef = 1;
+  p->xFree = 0;
+  p->pAttach = 0;
+#ifdef SQLITE_DEBUG
+  p->uMagic = SQLITE_RCSTR_MAGIC;
+#endif
+  return (char*)&p[1];
+}
+
+/*
+** Return the number of bytes allocated to the string.  The value returned
+** does not include the space for the zero-terminator at the end.
+*/
+u64 sqlite3RCStrSize(char *z){
+  RCStr *p = (RCStr*)z;
+  u64 N;
+  assert( p!=0 );
+  p--;
+  assert( p->nRCRef>0 );
+  assert( p->uMagic==SQLITE_RCSTR_MAGIC );
+  N = sqlite3_msize(p);
+  N -= sizeof(p) + 1;
+  return N;
+}
+
+/*
+** Change the size of the string so that it is able to hold N bytes.
+** The string might be reallocated, so return the new allocation.
+*/
+char *sqlite3RCStrResize(char *z, u64 N){
+  RCStr *p = (RCStr*)z;
+  RCStr *pNew;
+  assert( p!=0 );
+  p--;
+  assert( p->nRCRef==1 );
+  assert( p->uMagic==SQLITE_RCSTR_MAGIC );
+  pNew = sqlite3_realloc64(p, N+sizeof(RCStr)+1);
+  if( pNew==0 ){
+    sqlite3_free(p);
+    return 0;
+  }else{
+    return (char*)&pNew[1];
+  }
+}
+
+/*
+** Add a new attachment to the string.
+**
+** A string may have no more than one attachment.  When a new attachment
+** is added, any prior attachment is destroyed.  Remove an attachment
+** by adding a zero-attachment.
+*/
+void sqlite3RCStrAttach(char *z, void *pAttach, void(*xFree)(void*)){
+  RCStr *p = (RCStr*)z;
+  assert( p!=0 );
+  p--;
+  assert( p->nRCRef>0 );
+  assert( p->uMagic==SQLITE_RCSTR_MAGIC );
+  if( p->xFree ) p->xFree(p->pAttach);
+  p->xFree = xFree;
+  p->pAttach = pAttach;
+}
+  
+/*
+** Return the attachment associated with a string if the attachment
+** has the destructure specified in the second argument.  If the
+** string has no attachment or if the destructor does not match,
+** then return a NULL pointr.
+*/
+void *sqlite3RCStrGetAttachment(char *z, void(*xFree)(void*)){
+  RCStr *p = (RCStr*)z;
+  assert( p!=0 );
+  p--;
+  assert( p->nRCRef>0 );
+  assert( p->uMagic==SQLITE_RCSTR_MAGIC );
+  return p->xFree==xFree ? p->pAttach : 0;
+}
diff --git a/src/sqliteInt.h b/src/sqliteInt.h
index f214862f7..adf4d34de 100644
--- a/src/sqliteInt.h
+++ b/src/sqliteInt.h
@@ -1281,6 +1281,7 @@ typedef struct Parse Parse;
 typedef struct ParseCleanup ParseCleanup;
 typedef struct PreUpdate PreUpdate;
 typedef struct PrintfArguments PrintfArguments;
+typedef struct RCStr RCStr;
 typedef struct RenameToken RenameToken;
 typedef struct Returning Returning;
 typedef struct RowSet RowSet;
@@ -4061,6 +4062,47 @@ struct sqlite3_str {
 
 #define isMalloced(X)  (((X)->printfFlags & SQLITE_PRINTF_MALLOCED)!=0)
 
+/*
+** The following object is the header for an "RCStr" or "reference-counted
+** string".  An RCStr is passed around and used like any other char*
+** that has been dynamically allocated.  The important interface
+** difference is that it uses sqlite3RCStrUnref() as its destructor
+** rather than sqlite3_free().  Other than that the two are interchangeable.
+**
+** Thus to return an RCStr object as the result of an SQL function use:
+** 
+**    sqlite3_result_text64(ctx,z,sz,sqlite3RCStrUnref,SQLITE_UTF8)
+**                                   ^^^^^^^^^^^^^^^^^
+**                                   Instead of sqlite3_free() or similar
+**
+** An SQL function can check its arguments to see if they are RCStr
+** strings using the sqlite3ValueIsOfClass() function:
+**
+**    sqlite3ValueIsOfClass(argv[i], sqlite3RCStrUnref);
+**
+** An RCStr string might be better than an ordinary string in some cases
+** because:
+**
+**    (1)  You can duplicate it using sqlite3RCStrRef(x).
+**
+**    (2)  You can also add an associated object to the string.  For
+**         example, if the string is JSON, perhaps the associated object
+**         is a parse of that JSON.
+**
+** Methods for an RCStr string begin with "sqlite3RCStr...".
+*/
+struct RCStr {
+  u32 nRCRef;            /* Number of references */
+#ifdef SQLITE_DEBUG
+  u32 uMagic;            /* Magic number for sanity checking */
+#endif
+  void *pAttach;         /* Attachment to this string */
+  void (*xFree)(void*);  /* Destructor for the attachment */
+};
+
+/* The Magic number used by RCStr for sanity checking.  SQLITE_DEBUG only. */
+#define SQLITE_RCSTR_MAGIC 0x3dc05d54
+
 
 /*
 ** A pointer to this structure is used to communicate information
@@ -5180,6 +5222,7 @@ void sqlite3FileSuffix3(const char*, char*);
 u8 sqlite3GetBoolean(const char *z,u8);
 
 const void *sqlite3ValueText(sqlite3_value*, u8);
+int sqlite3ValueIsOfClass(const sqlite3_value*, void(*)(void*));
 int sqlite3ValueBytes(sqlite3_value*, u8);
 void sqlite3ValueSetStr(sqlite3_value*, int, const void *,u8,
                         void(*)(void*));
@@ -5287,6 +5330,15 @@ void sqlite3OomClear(sqlite3*);
 int sqlite3ApiExit(sqlite3 *db, int);
 int sqlite3OpenTempDatabase(Parse *);
 
+char *sqlite3RCStrRef(char*);
+void sqlite3RCStrUnref(char*);
+char *sqlite3RCStrNew(u64);
+u64 sqlite3RCStrSize(char*);
+char *sqlite3RCStrResize(char*,u64);
+int sqlite3RCStrIsWriteable(char*);
+void sqlite3RCStrAttach(char*, void*, void(*)(void*));
+void *sqlite3RCStrGetAttachment(char*,void(*)(void*));
+
 void sqlite3StrAccumInit(StrAccum*, sqlite3*, char*, int, int);
 int sqlite3StrAccumEnlarge(StrAccum*, i64);
 char *sqlite3StrAccumFinish(StrAccum*);
diff --git a/src/vdbemem.c b/src/vdbemem.c
index b5a794ae8..87dfbbebd 100644
--- a/src/vdbemem.c
+++ b/src/vdbemem.c
@@ -333,6 +333,11 @@ void sqlite3VdbeMemZeroTerminateIfAble(Mem *pMem){
       pMem->flags |= MEM_Term;
       return;
     }
+    if( pMem->xDel==(void(*)(void*))sqlite3RCStrUnref ){
+      /* Blindly assume that all RCStr objects are zero-terminated */
+      pMem->flags |= MEM_Term;
+      return;
+    }
   }else if( pMem->szMalloc>0 && pMem->szMalloc >= pMem->n+1 ){
     pMem->z[pMem->n] = 0;
     pMem->flags |= MEM_Term;
@@ -1363,6 +1368,24 @@ const void *sqlite3ValueText(sqlite3_value* pVal, u8 enc){
   return valueToText(pVal, enc);
 }
 
+/* Return true if sqlit3_value object pVal is a string or blob value
+** that uses the destructor specified in the second argument.
+**
+** TODO:  Maybe someday promote this interface into a published API so
+** that third-party extensions can get access to it?
+*/
+int sqlite3ValueIsOfClass(const sqlite3_value *pVal, void(*xFree)(void*)){
+  if( ALWAYS(pVal!=0)
+   && (pVal->flags & (MEM_Str|MEM_Blob))!=0
+   && (pVal->flags & MEM_Dyn)!=0
+   && pVal->xDel==xFree
+  ){
+    return 1;
+  }else{
+    return 0;
+  }
+}
+
 /*
 ** Create a new sqlite3_value object.
 */