nim-gdb.py fixes mostly for nimsuggest debugging (#16479)

These fixes were primarily developed to assist in nimsuggest debugging. There
is nothing intentionally specific done for nimsuggest, but beyond the automated
tests all practical testing was done with nimsuggest. Undoubltedly these will
also assist in other debugging scenarios.

The current nim-dbg.py script was broken in a few ways:
- failed to provide detailed value information for common types (see below)
- was not passing existing tests
- could not produce type summary information

Broken types now working somewhat better:
- sequences with ref types like strings
- sequences with value types like ints
- arrays with ref types like strings
- tables with int or string keys

Other improvements:
- slightly more test coverage

Future considerations:
- this, data used by it, should be something the compiler can generates
- account for different memory layouts ([arc/orc differ](https://github.com/nim-lang/Nim/pull/16479#issuecomment-751469536))

Attempts at improving nim-gdb.py

More tests, few fixes for seq and type printing

Tables debugging fixed added further tests

Fixed type printing

3 files changed, 157 insertions, 77 deletions

diff --git a/tests/untestable/gdb/gdb_pretty_printer_test.py b/tests/untestable/gdb/gdb_pretty_printer_test.py
index f002941ec..5b34bcb3d 100644
--- a/tests/untestable/gdb/gdb_pretty_printer_test.py
+++ b/tests/untestable/gdb/gdb_pretty_printer_test.py
@@ -6,31 +6,47 @@ import gdb
 # frontends might still be broken.
 
 gdb.execute("source ../../../tools/nim-gdb.py")
-# debug all instances of the generic function `myDebug`, should be 8
+# debug all instances of the generic function `myDebug`, should be 14
 gdb.execute("rbreak myDebug")
 gdb.execute("run")
 
 outputs = [
   'meTwo',
+  '""',
   '"meTwo"',
   '{meOne, meThree}',
   'MyOtherEnum(1)',
   '5',
   'array = {1, 2, 3, 4, 5}',
+  'seq(0, 0)',
+  'seq(0, 10)',
+  'array = {"one", "two"}',
+  'seq(3, 3) = {1, 2, 3}',
   'seq(3, 3) = {"one", "two", "three"}',
-  'Table(3, 64) = {["two"] = 2, ["three"] = 3, ["one"] = 1}',
+  'Table(3, 64) = {[4] = "four", [5] = "five", [6] = "six"}',
+  'Table(3, 8) = {["two"] = 2, ["three"] = 3, ["one"] = 1}',
 ]
 
 for i, expected in enumerate(outputs):
+  gdb.write(f"{i+1}) expecting: {expected}: ", gdb.STDLOG)
+  gdb.flush()
+
   functionSymbol = gdb.selected_frame().block().function
   assert functionSymbol.line == 21
 
-  if i == 5:
+  if i == 6:
     # myArray is passed as pointer to int to myDebug. I look up myArray up in the stack
     gdb.execute("up")
-    output = str(gdb.parse_and_eval("myArray"))
+    raw = gdb.parse_and_eval("myArray")    
+  elif i == 9:
+    # myOtherArray is passed as pointer to int to myDebug. I look up myOtherArray up in the stack
+    gdb.execute("up")
+    raw = gdb.parse_and_eval("myOtherArray")
   else:
-    output = str(gdb.parse_and_eval("arg"))
+    raw = gdb.parse_and_eval("arg")
+
+  output = str(raw)
 
   assert output == expected, output + " != " + expected
+  gdb.write(f"passed\n", gdb.STDLOG)
   gdb.execute("continue")
diff --git a/tests/untestable/gdb/gdb_pretty_printer_test_program.nim b/tests/untestable/gdb/gdb_pretty_printer_test_program.nim
index 458435c1a..c376ef89a 100644
--- a/tests/untestable/gdb/gdb_pretty_printer_test_program.nim
+++ b/tests/untestable/gdb/gdb_pretty_printer_test_program.nim
@@ -23,29 +23,56 @@ proc myDebug[T](arg: T): void =
 
 proc testProc(): void =
   var myEnum = meTwo
-  myDebug(myEnum)
+  myDebug(myEnum) #1
+  
+  # create a string, but don't allocate it
+  var myString: string
+  myDebug(myString) #2
+
   # create a string object but also make the NTI for MyEnum is generated
-  var myString = $myEnum
-  myDebug(myString)
+  myString = $myEnum
+  myDebug(myString) #3
+  
   var mySet = {meOne,meThree}
-  myDebug(mySet)
+  myDebug(mySet) #4
 
   # for MyOtherEnum there is no NTI. This tests the fallback for the pretty printer.
   var moEnum = moTwo
-  myDebug(moEnum)
+  myDebug(moEnum) #5
+
   var moSet = {moOne,moThree}
-  myDebug(moSet)
+  myDebug(moSet) #6
 
   let myArray = [1,2,3,4,5]
-  myDebug(myArray)
-  let mySeq   = @["one","two","three"]
-  myDebug(mySeq)
-
-  var myTable = initTable[string, int]()
-  myTable["one"] = 1
-  myTable["two"] = 2
-  myTable["three"] = 3
-  myDebug(myTable)
+  myDebug(myArray) #7
+
+  # implicitly initialized seq test
+  var mySeq: seq[string]
+  myDebug(mySeq) #8
+
+  # len not equal to capacity
+  let myOtherSeq = newSeqOfCap[string](10)
+  myDebug(myOtherSeq) #9
+
+  let myOtherArray = ["one","two"]
+  myDebug(myOtherArray) #10
+
+  # numeric sec
+  var mySeq3 = @[1,2,3]
+  myDebug(mySeq3) #11
+
+  # seq had to grow
+  var mySeq4 = @["one","two","three"]
+  myDebug(mySeq4) #12
+
+  var myTable = initTable[int, string]()
+  myTable[4] = "four"
+  myTable[5] = "five"
+  myTable[6] = "six"
+  myDebug(myTable) #13
+
+  var myOtherTable = {"one": 1, "two": 2, "three": 3}.toTable
+  myDebug(myOtherTable) #14
 
   echo(counter)
 
diff --git a/tools/nim-gdb.py b/tools/nim-gdb.py
index e994531b6..8143b94d5 100644
--- a/tools/nim-gdb.py
+++ b/tools/nim-gdb.py
@@ -34,6 +34,14 @@ def getNimRti(type_name):
     except:
       return None
 
+def getNameFromNimRti(rti_val):
+  """ Return name (or None) given a Nim RTI ``gdb.Value`` """
+  try:
+    # sometimes there isn't a name field -- example enums
+    return rti['name'].string(encoding="utf-8", errors="ignore")
+  except:
+    return None
+
 class NimTypeRecognizer:
   # this type map maps from types that are generated in the C files to
   # how they are called in nim. To not mix up the name ``int`` from
@@ -42,30 +50,25 @@ class NimTypeRecognizer:
   # ``int``.
 
   type_map_static = {
-    'NI': 'system.int',  'NI8': 'int8', 'NI16': 'int16',  'NI32': 'int32',  'NI64': 'int64',
-    'NU': 'uint', 'NU8': 'uint8','NU16': 'uint16', 'NU32': 'uint32', 'NU64': 'uint64',
+    'NI': 'system.int',  'NI8': 'int8', 'NI16': 'int16',  'NI32': 'int32',
+    'NI64': 'int64',
+    
+    'NU': 'uint', 'NU8': 'uint8','NU16': 'uint16', 'NU32': 'uint32',
+    'NU64': 'uint64',
+    
     'NF': 'float', 'NF32': 'float32', 'NF64': 'float64',
-    'NIM_BOOL': 'bool', 'NIM_CHAR': 'char', 'NCSTRING': 'cstring',
-    'NimStringDesc': 'string'
+    
+    'NIM_BOOL': 'bool',
+
+    'NIM_CHAR': 'char', 'NCSTRING': 'cstring', 'NimStringDesc': 'string'
   }
 
-  # Normally gdb distinguishes between the command `ptype` and
-  # `whatis`.  `ptype` prints a very detailed view of the type, and
-  # `whatis` a very brief representation of the type. I haven't
-  # figured out a way to know from the type printer that is
-  # implemented here how to know if a type printer should print the
-  # short representation or the long representation.  As a hacky
-  # workaround I just say I am not resposible for printing pointer
-  # types (seq and string are exception as they are semantically
-  # values).  this way the default type printer will handle pointer
-  # types and dive into the members of that type.  So I can still
-  # control with `ptype myval` and `ptype *myval` if I want to have
-  # detail or not.  I this this method stinks but I could not figure
-  # out a better solution.
-
-  object_type_pattern = re.compile("^(\w*):ObjectType$")
+  # object_type_pattern = re.compile("^(\w*):ObjectType$")
 
   def recognize(self, type_obj):
+    # skip things we can't handle like functions
+    if type_obj.code in [gdb.TYPE_CODE_FUNC, gdb.TYPE_CODE_VOID]:
+      return None
 
     tname = None
     if type_obj.tag is not None:
@@ -75,44 +78,43 @@ class NimTypeRecognizer:
 
     # handle pointer types
     if not tname:
-      if type_obj.code == gdb.TYPE_CODE_PTR:
+      target_type = type_obj
+      if type_obj.code in [gdb.TYPE_CODE_PTR]:
         target_type = type_obj.target()
-        target_type_name = target_type.name
-        if target_type_name:
-          # visualize 'string' as non pointer type (unpack pointer type).
-          if target_type_name == "NimStringDesc":
-            tname = target_type_name # could also just return 'string'
-          # visualize 'seq[T]' as non pointer type.
-          if target_type_name.find('tySequence_') == 0:
-            tname = target_type_name
 
-    if not tname:
-      # We are not resposible for this type printing.
-      # Basically this means we don't print pointer types.
-      return None
+      if target_type.name:
+        # visualize 'string' as non pointer type (unpack pointer type).
+        if target_type.name == "NimStringDesc":
+          tname = target_type.name # could also just return 'string'
+        else:
+          rti = getNimRti(target_type.name)
+          if rti:
+            return getNameFromNimRti(rti)
 
-    result = self.type_map_static.get(tname, None)
-    if result:
-      return result
+    if tname:
+      result = self.type_map_static.get(tname, None)
+      if result:
+        return result
 
-    rti = getNimRti(tname)
-    if rti:
-      return rti['name'].string("utf-8", "ignore")
-    else:
-      return None
+      rti = getNimRti(tname)
+      if rti:
+        return getNameFromNimRti(rti)
+
+    return None
 
 class NimTypePrinter:
   """Nim type printer. One printer for all Nim types."""
 
-
   # enabling and disabling of type printers can be done with the
   # following gdb commands:
   #
   #   enable  type-printer NimTypePrinter
   #   disable type-printer NimTypePrinter
+  # relevant docs: https://sourceware.org/gdb/onlinedocs/gdb/Type-Printing-API.html
 
   name = "NimTypePrinter"
-  def __init__ (self):
+
+  def __init__(self):
     self.enabled = True
 
   def instantiate(self):
@@ -309,9 +311,25 @@ class NimStringPrinter:
   def to_string(self):
     if self.val:
       l = int(self.val['Sup']['len'])
-      return self.val['data'][0].address.string("utf-8", "ignore", l)
+      return self.val['data'].lazy_string(encoding="utf-8", length=l)
     else:
-     return ""
+      return ""
+
+# class NimStringPrinter:
+#   pattern = re.compile(r'^NimStringDesc$')
+
+#   def __init__(self, val):
+#     self.val = val
+  
+#   def display_hint(self):
+#     return 'string'
+  
+#   def to_string(self):
+#     if self.val:
+#       l = int(self.val['Sup']['len'])
+#       return self.val['data'].lazy_string(encoding="utf-8", length=l)
+#     else:
+#       return ""
 
 class NimRopePrinter:
   pattern = re.compile(r'^tyObject_RopeObj__([A-Za-z0-9]*) \*$')
@@ -372,14 +390,15 @@ class NimEnumPrinter:
   pattern = re.compile(r'^tyEnum_(\w*)__([A-Za-z0-9]*)$')
 
   def __init__(self, val):
-    self.val      = val
-    match = self.pattern.match(self.val.type.name)
+    self.val = val
+    typeName = self.val.type.name
+    match = self.pattern.match(typeName)
     self.typeNimName  = match.group(1)
     typeInfoName = "NTI__" + match.group(2) + "_"
     self.nti = gdb.lookup_global_symbol(typeInfoName)
 
     if self.nti is None:
-      printErrorOnce(typeInfoName, "NimEnumPrinter: lookup global symbol '" + typeInfoName + " failed for " + self.val.type.name + ".\n")
+      printErrorOnce(typeInfoName, f"NimEnumPrinter: lookup global symbol '{typeInfoName}' failed for {typeName}.\n")
 
   def to_string(self):
     if self.nti:
@@ -476,11 +495,31 @@ class NimSeqPrinter:
 
   def children(self):
     if self.val:
-      length = int(self.val['Sup']['len'])
-      #align = len(str(length - 1))
-      for i in range(length):
-        yield ("data[{0}]".format(i), self.val["data"][i])
+      val = self.val
+      valType = val.type
+      length = int(val['Sup']['len'])
 
+      if length <= 0:
+        return
+
+      dataType = valType['data'].type
+      data = val['data']
+
+      if self.val.type.name is None:
+        dataType = valType['data'].type.target().pointer()
+        data = val['data'].cast(dataType)
+
+      inaccessible = False
+      for i in range(length):
+        if inaccessible:
+          return
+        try:
+          str(data[i])
+          yield "data[{0}]".format(i), data[i]
+        except RuntimeError:
+          inaccessible = True
+          yield "data[{0}]".format(i), "inaccessible"
+      
 ################################################################################
 
 class NimArrayPrinter:
@@ -524,9 +563,9 @@ class NimStringTablePrinter:
 
   def children(self):
     if self.val:
-      data = NimSeqPrinter(self.val['data'])
+      data = NimSeqPrinter(self.val['data'].dereference())
       for idxStr, entry in data.children():
-        if int(entry['Field2']) > 0:
+        if int(entry['Field0']) != 0:
           yield (idxStr + ".Field0", entry['Field0'])
           yield (idxStr + ".Field1", entry['Field1'])
 
@@ -537,7 +576,6 @@ class NimTablePrinter:
 
   def __init__(self, val):
     self.val = val
-    # match = self.pattern.match(self.val.type.name)
 
   def display_hint(self):
     return 'map'
@@ -556,11 +594,10 @@ class NimTablePrinter:
     if self.val:
       data = NimSeqPrinter(self.val['data'])
       for idxStr, entry in data.children():
-        if int(entry['Field0']) > 0:
+        if int(entry['Field0']) != 0:
           yield (idxStr + '.Field1', entry['Field1'])
           yield (idxStr + '.Field2', entry['Field2'])
 
-
 ################################################################
 
 # this is untested, therefore disabled
@@ -651,7 +688,7 @@ def register_nim_pretty_printers_for_object(objfile):
   if nimMainSym and nimMainSym.symtab.objfile == objfile:
     print("set Nim pretty printers for ", objfile.filename)
 
-    objfile.type_printers = [NimTypePrinter()]
+    gdb.types.register_type_printer(objfile, NimTypePrinter())
     objfile.pretty_printers = [makematcher(var) for var in list(globals().values()) if hasattr(var, 'pattern')]
 
 # Register pretty printers for all objfiles that are already loaded.