Improve the performance of the built-in REPLACE() function in cases where

it does many substitutions that make the string larger.  OSSFuzz is reporting
intermittant timeouts when running a test where it does a REPLACE() on a 
930KB random blob. Perhaps this enhancement will fix that.

FossilOrigin-Name: fab2c2b07b5d3cd851db3e6f5c8a44155e32b0df22905ea33412b153b825a928

1 files changed, 26 insertions, 17 deletions

diff --git a/src/func.c b/src/func.c
index 4e80535e8..399f6dba3 100644
--- a/src/func.c
+++ b/src/func.c
@@ -1199,6 +1199,8 @@ static void replaceFunc(
   i64 nOut;                /* Maximum size of zOut */
   int loopLimit;           /* Last zStr[] that might match zPattern[] */
   int i, j;                /* Loop counters */
+  unsigned cntExpand;      /* Number zOut expansions */
+  sqlite3 *db = sqlite3_context_db_handle(context);
 
   assert( argc==3 );
   UNUSED_PARAMETER(argc);
@@ -1230,33 +1232,40 @@ static void replaceFunc(
     return;
   }
   loopLimit = nStr - nPattern;  
+  cntExpand = 0;
   for(i=j=0; i<=loopLimit; i++){
     if( zStr[i]!=zPattern[0] || memcmp(&zStr[i], zPattern, nPattern) ){
       zOut[j++] = zStr[i];
     }else{
-      u8 *zOld;
-      sqlite3 *db = sqlite3_context_db_handle(context);
-      nOut += nRep - nPattern;
-      testcase( nOut-1==db->aLimit[SQLITE_LIMIT_LENGTH] );
-      testcase( nOut-2==db->aLimit[SQLITE_LIMIT_LENGTH] );
-      if( nOut-1>db->aLimit[SQLITE_LIMIT_LENGTH] ){
-        sqlite3_result_error_toobig(context);
-        sqlite3_free(zOut);
-        return;
-      }
-      zOld = zOut;
-      zOut = sqlite3_realloc64(zOut, (int)nOut);
-      if( zOut==0 ){
-        sqlite3_result_error_nomem(context);
-        sqlite3_free(zOld);
-        return;
+      if( nRep>nPattern ){
+        nOut += nRep - nPattern;
+        if( nOut-1>db->aLimit[SQLITE_LIMIT_LENGTH] ){
+          sqlite3_result_error_toobig(context);
+          sqlite3_free(zOut);
+          return;
+        }
+        testcase( nOut-1==db->aLimit[SQLITE_LIMIT_LENGTH] );
+        testcase( nOut-2==db->aLimit[SQLITE_LIMIT_LENGTH] );
+        cntExpand++;
+        if( (cntExpand&(cntExpand-1))==0 ){
+          /* Grow the size of the output buffer only on substitutions
+          ** whose index is a power of two: 1, 2, 4, 8, 16, 32, ... */
+          u8 *zOld;
+          zOld = zOut;
+          zOut = sqlite3_realloc64(zOut, (int)nOut + (nOut - nStr - 1));
+          if( zOut==0 ){
+            sqlite3_result_error_nomem(context);
+            sqlite3_free(zOld);
+            return;
+          }
+        }
       }
       memcpy(&zOut[j], zRep, nRep);
       j += nRep;
       i += nPattern-1;
     }
   }
-  assert( j+nStr-i+1==nOut );
+  assert( j+nStr-i+1<=nOut );
   memcpy(&zOut[j], &zStr[i], nStr-i);
   j += nStr - i;
   assert( j<=nOut );