Fix appender code that might not initialize memory that might point a…

…t huge allocations (#9084)

std/array.d

@@ -3607,6 +3607,7 @@ if (isDynamicArray!A)
         }
         else
         {
+            import core.stdc.string : memcpy, memset;
             // Time to reallocate.
             // We need to almost duplicate what's in druntime, except we
             // have better access to the capacity field.
@@ -3618,6 +3619,15 @@ if (isDynamicArray!A)
                 if (u)
                 {
                     // extend worked, update the capacity
+                    // if the type has indirections, we need to zero any new
+                    // data that we requested, as the existing data may point
+                    // at large unused blocks.
+                    static if (hasIndirections!T)
+                    {
+                        immutable addedSize = u - (_data.capacity * T.sizeof);
+                        () @trusted { memset(_data.arr.ptr + _data.capacity, 0, addedSize); }();
+                    }
+
                     _data.capacity = u / T.sizeof;
                     return;
                 }
@@ -3633,10 +3643,20 @@ if (isDynamicArray!A)
 
             auto bi = (() @trusted => GC.qalloc(nbytes, blockAttribute!T))();
             _data.capacity = bi.size / T.sizeof;
-            import core.stdc.string : memcpy;
             if (len)
                 () @trusted { memcpy(bi.base, _data.arr.ptr, len * T.sizeof); }();
+
             _data.arr = (() @trusted => (cast(Unqual!T*) bi.base)[0 .. len])();
+
+            // we requested new bytes that are not in the existing
+            // data. If T has pointers, then this new data could point at stale
+            // objects from the last time this block was allocated. Zero that
+            // new data out, it may point at large unused blocks!
+            static if (hasIndirections!T)
+                () @trusted {
+                    memset(bi.base + (len * T.sizeof), 0, (newlen - len) * T.sizeof);
+                }();
+
             _data.tryExtendBlock = true;
             // leave the old data, for safety reasons
         }
@@ -4011,6 +4031,43 @@ if (isDynamicArray!A)
     app2.toString();
 }
 
+// https://issues.dlang.org/show_bug.cgi?id=24856
+@system unittest
+{
+    import core.memory : GC;
+    import std.stdio : writeln;
+    import std.algorithm.searching : canFind;
+    GC.disable();
+    scope(exit) GC.enable();
+    void*[] freeme;
+    // generate some poison blocks to allocate from.
+    auto poison = cast(void*) 0xdeadbeef;
+    foreach (i; 0 .. 10)
+    {
+        auto blk = new void*[7];
+        blk[] = poison;
+        freeme ~= blk.ptr;
+    }
+
+    foreach (p; freeme)
+        GC.free(p);
+
+    int tests = 0;
+    foreach (i; 0 .. 10)
+    {
+        Appender!(void*[]) app;
+        app.put(null);
+        // if not a realloc of one of the deadbeef pointers, continue
+        if (!freeme.canFind(app.data.ptr))
+            continue;
+        ++tests;
+        assert(!app.data.ptr[0 .. app.capacity].canFind(poison), "Appender not zeroing data!");
+    }
+    // just notify in the log whether this test actually could be done.
+    if (tests == 0)
+        writeln("WARNING: test of Appender zeroing did not occur");
+}
+
 //Calculates an efficient growth scheme based on the old capacity
 //of data, and the minimum requested capacity.
 //arg curLen: The current length