added logic to the M&S GC to detect logical leaks

lib/system/gc_common.nim

@@ -25,6 +25,13 @@ when defined(nimTypeNames):
         c_fprintf(stdout, "[Heap] %s: #%ld; bytes: %ld\n", it.name, it.instances, it.sizes)
       it = it.nextType
 
+  when defined(nimGcRefLeak):
+    proc oomhandler() =
+      c_fprintf(stdout, "[Heap] ROOTS: #%ld\n", gch.additionalRoots.len)
+      writeLeaks()
+
+    outOfMemHook = oomhandler
+
 template decTypeSize(cell, t) =
   # XXX this needs to use atomics for multithreaded apps!
   when defined(nimTypeNames):

lib/system/gc_ms.nim

@@ -142,11 +142,54 @@ proc doOperation(p: pointer, op: WalkOp) {.benign.}
 proc forAllChildrenAux(dest: pointer, mt: PNimType, op: WalkOp) {.benign.}
 # we need the prototype here for debugging purposes
 
+when defined(nimGcRefLeak):
+  const
+    MaxTraceLen = 20 # tracking the last 20 calls is enough
+
+  type
+    GcStackTrace = object
+      lines: array[0..MaxTraceLen-1, cstring]
+      files: array[0..MaxTraceLen-1, cstring]
+
+  proc captureStackTrace(f: PFrame, st: var GcStackTrace) =
+    const
+      firstCalls = 5
+    var
+      it = f
+      i = 0
+      total = 0
+    while it != nil and i <= high(st.lines)-(firstCalls-1):
+      # the (-1) is for the "..." entry
+      st.lines[i] = it.procname
+      st.files[i] = it.filename
+      inc(i)
+      inc(total)
+      it = it.prev
+    var b = it
+    while it != nil:
+      inc(total)
+      it = it.prev
+    for j in 1..total-i-(firstCalls-1):
+      if b != nil: b = b.prev
+    if total != i:
+      st.lines[i] = "..."
+      st.files[i] = "..."
+      inc(i)
+    while b != nil and i <= high(st.lines):
+      st.lines[i] = b.procname
+      st.files[i] = b.filename
+      inc(i)
+      b = b.prev
+
+  var ax: array[10_000, GcStackTrace]
+
 proc nimGCref(p: pointer) {.compilerProc.} =
   # we keep it from being collected by pretending it's not even allocated:
   when false:
     when withBitvectors: excl(gch.allocated, usrToCell(p))
     else: usrToCell(p).refcount = rcBlack
+  when defined(nimGcRefLeak):
+    captureStackTrace(framePtr, ax[gch.additionalRoots.len])
   add(gch.additionalRoots, usrToCell(p))
 
 proc nimGCunref(p: pointer) {.compilerProc.} =
@@ -157,6 +200,8 @@ proc nimGCunref(p: pointer) {.compilerProc.} =
   while i >= 0:
     if d[i] == cell:
       d[i] = d[L]
+      when defined(nimGcRefLeak):
+        ax[i] = ax[L]
       dec gch.additionalRoots.len
       break
     dec(i)
@@ -164,6 +209,16 @@ proc nimGCunref(p: pointer) {.compilerProc.} =
     when withBitvectors: incl(gch.allocated, usrToCell(p))
     else: usrToCell(p).refcount = rcWhite
 
+when defined(nimGcRefLeak):
+  proc writeLeaks() =
+    for i in 0..gch.additionalRoots.len-1:
+      c_fprintf(stdout, "[Heap] NEW STACK TRACE\n")
+      for ii in 0..MaxTraceLen-1:
+        let line = ax[i].lines[ii]
+        let file = ax[i].files[ii]
+        if isNil(line): break
+        c_fprintf(stdout, "[Heap] %s(%s)\n", file, line)
+
 include gc_common
 
 proc prepareDealloc(cell: PCell) =