Hi, Sergey,

thanks for the patch! LGTM

This commit is the follow-up for the commit
593fef813b7d36060e0c1b1ae2df0fbc0d604d1f ("lldb: refactor extension").
`GetValueForExpressionPath()` may produce the empty value for negative
indexes. This leads to incorrect frame unwinding in the LLDB extension
and errors like:
| Failed to execute command `lj-stack`:
| 'Q' format requires 0 <= number <= 18446744073709551615

This patch adds special handling for the negative indices.
---
 src/luajit_dbg.py                             | 49 ++++++++++++++++---
 .../debug-extension-tests.py                  | 26 +++++++---
 2 files changed, 62 insertions(+), 13 deletions(-)

diff --git a/src/luajit_dbg.py b/src/luajit_dbg.py
index d9196f06..410f0191 100644
--- a/src/luajit_dbg.py
+++ b/src/luajit_dbg.py
@@ -255,15 +255,31 @@ class _GDBDebugger(Debugger):
 
 class _LLDBDebugger(Debugger):
     def _lldb_tp_isfp(self, tp):
-        return tp.GetBasicType() in [
+        return tp.GetCanonicalType().GetBasicType() in [
             lldb.eBasicTypeFloat,
             lldb.eBasicTypeDouble,
             lldb.eBasicTypeLongDouble
         ]
 
+    def _lldb_tp_issigned(self, tp):
+        return tp.GetCanonicalType().GetBasicType() in [
+            lldb.eBasicTypeChar,
+            lldb.eBasicTypeSignedChar,
+            lldb.eBasicTypeShort,
+            lldb.eBasicTypeInt,
+            lldb.eBasicTypeLong,
+            lldb.eBasicTypeLongLong,
+            lldb.eBasicTypeInt128
+        ]
+
     def _lldb_value_from_raw(self, raw_value, size, tp):
         isfp = self._lldb_tp_isfp(tp)
-        pack_flag = '<d' if isfp else '<Q'
+        if isfp:
+            pack_flag = '<d'
+        elif self._lldb_tp_issigned(tp):
+            pack_flag = '<q'
+        else:
+            pack_flag = '<Q'
         raw_data = struct.pack(pack_flag, raw_value)
         sbdata = lldb.SBData()
         sbdata.SetData(
@@ -308,9 +324,24 @@ class _LLDBDebugger(Debugger):
                 key = int(key)
             if type(key) is int:
                 # Allow array access.
-                return lldb.value(
-                    lldbval.sbvalue.GetValueForExpressionPath('[%i]' % key)
-                )
+                if key >= 0:
+                    return lldb.value(
+                        lldbval.sbvalue.GetValueForExpressionPath('[%i]' % key)
+                    )
+                else:
+                    # GetValueForExpressionPath doesn't work for
+                    # negative offsets.
+                    sbvalue = lldbval.sbvalue
+                    assert sbvalue.TypeIsPointerType(), \
+                        'attempt to get index of non-pointer type'
+                    tp = sbvalue.GetType().GetPointeeType()
+                    sz = sbvalue.deref.size
+                    addr = sbvalue.GetValueAsUnsigned() + key * sz
+                    return lldb.value(self.target.CreateValueFromAddress(
+                            '({tp}){addr}'.format(tp=tp, addr=addr),
+                            lldb.SBAddress(addr, self.target),
+                            tp,
+                    ))
             elif type(key) is str:
                 return lldb.value(lldbval.sbvalue.GetChildMemberWithName(key))
             raise Exception(TypeError('No item of type %s' % str(type(key))))
@@ -417,8 +448,12 @@ class _LLDBDebugger(Debugger):
         # may take the 8 bytes of memory instead of 4, before the
         # cast. Construct the value on the fly.
         tp = self._dbgtype(typestr)
-        is_fp = self._lldb_tp_isfp(tp)
-        rawval = float(val.GetValue()) if is_fp else val.GetValueAsUnsigned()
+        if self._lldb_tp_isfp(tp):
+            rawval = float(val.GetValue())
+        elif self._lldb_tp_issigned(tp):
+            rawval = val.GetValueAsSigned()
+        else:
+            rawval = val.GetValueAsUnsigned()
         return self._lldb_value_from_raw(rawval, val.GetByteSize(), tp)
 
     def sizeof(self, typestr):
diff --git a/test/tarantool-debugger-tests/debug-extension-tests.py b/test/tarantool-debugger-tests/debug-extension-tests.py
index 61529561..7cb60d84 100644
--- a/test/tarantool-debugger-tests/debug-extension-tests.py
+++ b/test/tarantool-debugger-tests/debug-extension-tests.py
@@ -186,16 +186,30 @@ class TestLJGC(TestCaseBase):
     )
 
 
-class TestLJStack(TestCaseBase):
+STACK_RX = (
+    r'-+ Red zone:\s+\d+ slots -+\n'
+    r'(' + RX_ADDR + r'\s+' + RX_FRAME + r' VALUE: nil\n?)*\n'
+    r'-+ Stack:\s+\d+ slots -+\n'
+    r'(' + RX_ADDR + r'(:' + RX_ADDR + r')?\s+' + RX_FRAME + r'.*\n?)+\n'
+)
+
+
+class TestLJStackBase(TestCaseBase):
     extension_cmds = 'lj-stack'
     location = 'lj_cf_print'
     lua_script = 'print(1)'
-    pattern = (
-        r'-+ Red zone:\s+\d+ slots -+\n'
-        r'(' + RX_ADDR + r'\s+' + RX_FRAME + r' VALUE: nil\n?)*\n'
-        r'-+ Stack:\s+\d+ slots -+\n'
-        r'(' + RX_ADDR + r'(:' + RX_ADDR + r')?\s+' + RX_FRAME + r'.*\n?)+\n'
+    pattern = STACK_RX
+
+
+# Check LLDB correctness for the specific stack.
+class TestLJStackFunc(TestCaseBase):
+    extension_cmds = 'lj-stack'
+    location = 'lj_cf_print'
+    lua_script = (
+        'local function nop() end\n'
+        'print()\n'
     )
+    pattern = STACK_RX
 
 
 class TestLJTV(TestCaseBase):