Fix std::fpos pretty printer on musl

[ccross]

Summary

The mbstate_t field in std::fpos is an opaque type provied by libc,
and musl's implementation does not match the one used by glibc.
Change StdFposPrinter to verify its assumptions about the layout
of mbstate_t, and leave out the state printing if it doesn't match.

[enh]

libcxx/utils/gdb/libcxx/printers.py

767

accidentally introduced tabs?

[ccross]

libcxx/utils/gdb/libcxx/printers.py

767

I'm not seeing any tabs in my local git CL, in the download raw diff here, or in the "show raw file (right)" output. I think that must be how phabricator highlights increased indentation?

[ldionne]

libcxx/utils/gdb/libcxx/printers.py

767

Tabs should be caught by our CI if introduced. Since it's green, I think it's all good.

libcxx/test/libcxx/gdb/gdb_pretty_printer_test.sh.cpp

ComparePrettyPrintToRegex(
       R"(std::shared_ptr<int> count [3\?], weak [0\?]( \(libc\+\+ missing debug info\))? containing = {__ptr_ = 0x[a-f0-9]+})");
 
   std::shared_ptr<const int> test3;
   ComparePrettyPrintToChars(test3, "std::shared_ptr is nullptr");
 }
 
 void streampos_test() {
   std::streampos test0 = 67;
-  ComparePrettyPrintToChars(
-      test0, "std::fpos with stream offset:67 with state: {count:0 value:0}");
+  ComparePrettyPrintToRegex(test0, "^std::fpos with stream offset:67( with state: {count:0 value:0})?$");
   std::istringstream input("testing the input stream here");
   std::streampos test1 = input.tellg();
-  ComparePrettyPrintToChars(
-      test1, "std::fpos with stream offset:0 with state: {count:0 value:0}");
+  ComparePrettyPrintToRegex(test1, "^std::fpos with stream offset:0( with state: {count:0 value:0})?$");
   std::unique_ptr<char[]> buffer(new char[5]);
   input.read(buffer.get(), 5);
   test1 = input.tellg();
-  ComparePrettyPrintToChars(
-      test1, "std::fpos with stream offset:5 with state: {count:0 value:0}");
+  ComparePrettyPrintToRegex(test1, "^std::fpos with stream offset:5( with state: {count:0 value:0})?$");
 }
 
 int main(int, char**) {
   framework_self_test();
 
   string_test();
   a_namespace::string_view_test();
 

libcxx/utils/gdb/libcxx/printers.py

class StdFposPrinter(object):
 
     def __init__(self, val):
         self.val = val
 
     def to_string(self):
         typename = _remove_generics(_prettify_typename(self.val.type))
         offset = self.val["__off_"]
         state = self.val["__st_"]
+
+        state_fields = []
+        if state.type.code == gdb.TYPE_CODE_STRUCT:
+            state_fields = [f.name for f in state.type.fields()]
+
+        state_string = ""
+        if "__count" in state_fields and "__value" in state_fields:
             count = state["__count"]
             value = state["__value"]["__wch"]
-        return "%s with stream offset:%s with state: {count:%s value:%s}" % (
-            typename, offset, count, value)
+            state_string = " with state: {count:%s value:%s}" % (count, value)
+
+        return "%s with stream offset:%s%s" % (typename, offset, state_string)
 
 
 class AbstractUnorderedCollectionPrinter(object):
     """Abstract super class for std::unordered_(multi)[set|map]."""
 
     def __init__(self, val):
         self.val = val
         self.table = val["__table_"]