[analyzer] Eliminate unique release point assertion (llvm#150240)

MallocChecker.cpp has a complex heuristic that supresses reports where
the memory release happens during the release of a reference-counted
object (to suppress a significant amount of false positives).

Previously this logic asserted that there is at most one release point
corresponding to a symbol, but it turns out that there is a rare corner
case where the symbol can be released, forgotten and then released
again. This commit removes that assertion to avoid the crash. (As this
issue just affects a bug suppression heuristic, I didn't want to dig
deeper and modify the way the state of the symbol is changed.)

Fixes llvm#149754

Author: NagyDonat

clang/lib/StaticAnalyzer/Checkers/MallocChecker.cpp

@@ -3730,13 +3730,15 @@ PathDiagnosticPieceRef MallocBugVisitor::VisitNode(const ExplodedNode *N,
           return nullptr;
         }
 
-        // Save the first destructor/function as release point.
-        assert(!ReleaseFunctionLC && "There should be only one release point");
+        // Record the stack frame that is _responsible_ for this memory release
+        // event. This will be used by the false positive suppression heuristics
+        // that recognize the release points of reference-counted objects.
+        //
+        // Usually (e.g. in C) we say that the _responsible_ stack frame is the
+        // current innermost stack frame:
         ReleaseFunctionLC = CurrentLC->getStackFrame();
-
-        // See if we're releasing memory while inlining a destructor that
-        // decrement reference counters (or one of its callees).
-        // This turns on various common false positive suppressions.
+        // ...but if the stack contains a destructor call, then we say that the
+        // outermost destructor stack frame is the _responsible_ one:
         for (const LocationContext *LC = CurrentLC; LC; LC = LC->getParent()) {
           if (const auto *DD = dyn_cast<CXXDestructorDecl>(LC->getDecl())) {
             if (isReferenceCountingPointerDestructor(DD)) {

clang/test/Analysis/malloc.c

@@ -1954,9 +1954,23 @@ int conjure(void);
 void testExtent(void) {
   int x = conjure();
   clang_analyzer_dump(x);
-  // expected-warning-re@-1 {{{{^conj_\$[[:digit:]]+{int, LC1, S[[:digit:]]+, #1}}}}}}
+  // expected-warning-re@-1 {{{{^conj_\$[[:digit:]]+{int, LC[[:digit:]]+, S[[:digit:]]+, #1}}}}}}
   int *p = (int *)malloc(x);
   clang_analyzer_dumpExtent(p);
-  // expected-warning-re@-1 {{{{^conj_\$[[:digit:]]+{int, LC1, S[[:digit:]]+, #1}}}}}}
+  // expected-warning-re@-1 {{{{^conj_\$[[:digit:]]+{int, LC[[:digit:]]+, S[[:digit:]]+, #1}}}}}}
   free(p);
 }
+
+void gh149754(void *p) {
+  // This testcase demonstrates an unusual situation where a certain symbol
+  // (the value of `p`) is released (more precisely, transitions from
+  // untracked state to Released state) twice within the same bug path because
+  // the `EvalAssume` callback resets it to untracked state after the first
+  // time when it is released. This caused the failure of an assertion, which
+  // was since then removed for the codebase.
+  if (!realloc(p, 8)) {
+    realloc(p, 8);
+    free(p); // expected-warning {{Attempt to free released memory}}
+  }
+  // expected-warning@+1 {{Potential memory leak}}
+}