[lldb/Core] Fix crash in ValueObject::CreateChildAtIndex

The patch fixes a crash in ValueObject::CreateChildAtIndex caused by a
null pointer dereferencing. This is a corner case that is happening when
trying to dereference a variable with an incomplete type, and this same
variable doesn't have a synthetic value to get the child ValueObject.

If this happens, lldb will now return a null pointer that will results
in an error message.

rdar://65181171

Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>

lldb/source/Core/ValueObject.cpp

@@ -687,10 +687,15 @@ ValueObject *ValueObject::CreateChildAtIndex(size_t idx,
         language_flags);
   }
 
-  if (!valobj && synthetic_array_member)
-    valobj = GetSyntheticValue()
-                 ->GetChildAtIndex(synthetic_index, synthetic_array_member)
-                 .get();
+  // In case of an incomplete type, LLDB will try to use the ValueObject's
+  // synthetic value to create the child ValueObject.
+  if (!valobj && synthetic_array_member) {
+    if (ValueObjectSP synth_valobj_sp = GetSyntheticValue()) {
+      valobj = synth_valobj_sp
+                   ->GetChildAtIndex(synthetic_index, synthetic_array_member)
+                   .get();
+    }
+  }
 
   return valobj;
 }

lldb/test/API/functionalities/target_var/Makefile

@@ -1,5 +1,3 @@
-include Makefile.rules
+C_SOURCES := main.c
 
-a.out: globals.ll
-	$(CC) $(CFLAGS) -g -c $^ -o globals.o
-	$(LD) $(LDFLAGS) -g globals.o -o $@
+include Makefile.rules

lldb/test/API/functionalities/target_var/TestTargetVar.py

@@ -20,3 +20,5 @@ def testTargetVarExpr(self):
         self.build()
         lldbutil.run_to_name_breakpoint(self, 'main')
         self.expect("target variable i", substrs=['i', '42'])
+        self.expect("target variable var", patterns=['\(incomplete \*\) var = 0[xX](0)*dead'])
+        self.expect("target variable var[0]", error=True, substrs=["can't find global variable 'var[0]'"])

lldb/test/API/functionalities/target_var/main.c

@@ -0,0 +1,7 @@
+int i = 42;
+int *p = &i;
+
+struct incomplete;
+struct incomplete *var = (struct incomplete *)0xdead;
+
+int main() { return *p; }