[lldb] Support alternatives for scope format entries

This PR implements support for specifying multiple alternatives for
scope format entries. Scopes are used to enclose things that should only
be printed when everything in the scope resolves.

For example, the following scope only resolves if both
`${line.file.basename}` and `${line.number}` resolve.
`

```
{ at ${line.file.basename}:${line.number}}
```

However, the current implementation doesn't let you specify what to
print when they don't resolve. This PR adds support for specifying
multiple alternative scopes, which are evaluated left-to-right.

For example:

```
{ at ${line.file.basename}:${line.number}| in ${function.name}| <unknown location>}
```

This will resolve to:

 - ` at ${line.file.basename}:${line.number}` if the corresponding variables resolve.
 - Otherwise, this resolves to ` in ${function.name}` if `${function.name}` resolves.
 - Otherwise, this resolves to ` <unknown location>` which always resolves.

This PR makes the `|` character a special character within a scope, but
allows it to be escaped.

I ended up with this approach because it fit quite nicely in the
existing architecture of the format entries and by limiting the
functionality to scopes, it sidesteps some complexity, like dealing with
recursion.

Author: JDevlieghere

lldb/include/lldb/Core/FormatEntity.h

@@ -11,10 +11,10 @@
 
 #include "lldb/lldb-enumerations.h"
 #include "lldb/lldb-types.h"
+#include "llvm/ADT/SmallVector.h"
 #include <algorithm>
 #include <cstddef>
 #include <cstdint>
-
 #include <string>
 #include <vector>
 
@@ -157,9 +157,7 @@ struct Entry {
   }
 
   Entry(Type t = Type::Invalid, const char *s = nullptr,
-        const char *f = nullptr)
-      : string(s ? s : ""), printf_format(f ? f : ""), type(t) {}
-
+        const char *f = nullptr);
   Entry(llvm::StringRef s);
   Entry(char ch);
 
@@ -169,15 +167,19 @@ struct Entry {
 
   void AppendText(const char *cstr);
 
-  void AppendEntry(const Entry &&entry) { children.push_back(entry); }
+  void AppendEntry(const Entry &&entry);
+
+  void StartAlternative();
 
   void Clear() {
     string.clear();
     printf_format.clear();
-    children.clear();
+    children_stack.clear();
+    children_stack.emplace_back();
     type = Type::Invalid;
     fmt = lldb::eFormatDefault;
     number = 0;
+    level = 0;
     deref = false;
   }
 
@@ -190,7 +192,7 @@ struct Entry {
       return false;
     if (printf_format != rhs.printf_format)
       return false;
-    if (children != rhs.children)
+    if (children_stack != rhs.children_stack)
       return false;
     if (type != rhs.type)
       return false;
@@ -201,9 +203,18 @@ struct Entry {
     return true;
   }
 
+  std::vector<Entry> &GetChildren();
+
   std::string string;
   std::string printf_format;
-  std::vector<Entry> children;
+
+  /// A stack of children entries, used by Scope entries to provide alterantive
+  /// children. All other entries have a stack of size 1.
+  /// @{
+  llvm::SmallVector<std::vector<Entry>, 1> children_stack;
+  size_t level = 0;
+  /// @}
+
   Type type;
   lldb::Format fmt = lldb::eFormatDefault;
   lldb::addr_t number = 0;

lldb/source/Core/FormatEntity.cpp

@@ -60,6 +60,7 @@
 #include "llvm/Support/Regex.h"
 #include "llvm/TargetParser/Triple.h"
 
+#include <cassert>
 #include <cctype>
 #include <cinttypes>
 #include <cstdio>
@@ -280,31 +281,53 @@ constexpr Definition g_top_level_entries[] = {
 constexpr Definition g_root = Entry::DefinitionWithChildren(
     "<root>", EntryType::Root, g_top_level_entries);
 
+FormatEntity::Entry::Entry(Type t, const char *s, const char *f)
+    : string(s ? s : ""), printf_format(f ? f : ""), children_stack({{}}),
+      type(t) {}
+
 FormatEntity::Entry::Entry(llvm::StringRef s)
-    : string(s.data(), s.size()), printf_format(), children(),
-      type(Type::String) {}
+    : string(s.data(), s.size()), children_stack({{}}), type(Type::String) {}
 
 FormatEntity::Entry::Entry(char ch)
-    : string(1, ch), printf_format(), children(), type(Type::String) {}
+    : string(1, ch), printf_format(), children_stack({{}}), type(Type::String) {
+}
+
+std::vector<Entry> &FormatEntity::Entry::GetChildren() {
+  assert(level < children_stack.size());
+  return children_stack[level];
+}
 
 void FormatEntity::Entry::AppendChar(char ch) {
-  if (children.empty() || children.back().type != Entry::Type::String)
-    children.push_back(Entry(ch));
+  auto &entries = GetChildren();
+  if (entries.empty() || entries.back().type != Entry::Type::String)
+    entries.push_back(Entry(ch));
   else
-    children.back().string.append(1, ch);
+    entries.back().string.append(1, ch);
 }
 
 void FormatEntity::Entry::AppendText(const llvm::StringRef &s) {
-  if (children.empty() || children.back().type != Entry::Type::String)
-    children.push_back(Entry(s));
+  auto &entries = GetChildren();
+  if (entries.empty() || entries.back().type != Entry::Type::String)
+    entries.push_back(Entry(s));
   else
-    children.back().string.append(s.data(), s.size());
+    entries.back().string.append(s.data(), s.size());
 }
 
 void FormatEntity::Entry::AppendText(const char *cstr) {
   return AppendText(llvm::StringRef(cstr));
 }
 
+void FormatEntity::Entry::AppendEntry(const Entry &&entry) {
+  auto &entries = GetChildren();
+  entries.push_back(entry);
+}
+
+void FormatEntity::Entry::StartAlternative() {
+  assert(type == Entry::Type::Scope);
+  children_stack.emplace_back();
+  level++;
+}
+
 #define ENUM_TO_CSTR(eee)                                                      \
   case FormatEntity::Entry::Type::eee:                                         \
     return #eee
@@ -403,8 +426,9 @@ void FormatEntity::Entry::Dump(Stream &s, int depth) const {
   if (deref)
     s.Printf("deref = true, ");
   s.EOL();
-  for (const auto &child : children) {
-    child.Dump(s, depth + 1);
+  for (const auto &children : children_stack) {
+    for (const auto &child : children)
+      child.Dump(s, depth + 1);
   }
 }
 
@@ -1306,7 +1330,7 @@ bool FormatEntity::Format(const Entry &entry, Stream &s,
     return true;
 
   case Entry::Type::Root:
-    for (const auto &child : entry.children) {
+    for (const auto &child : entry.children_stack[0]) {
       if (!Format(child, s, sc, exe_ctx, addr, valobj, function_changed,
                   initial_function)) {
         return false; // If any item of root fails, then the formatting fails
@@ -1320,19 +1344,26 @@ bool FormatEntity::Format(const Entry &entry, Stream &s,
 
   case Entry::Type::Scope: {
     StreamString scope_stream;
-    bool success = false;
-    for (const auto &child : entry.children) {
-      success = Format(child, scope_stream, sc, exe_ctx, addr, valobj,
-                       function_changed, initial_function);
-      if (!success)
-        break;
+    auto format_children = [&](const std::vector<Entry> &children) {
+      scope_stream.Clear();
+      for (const auto &child : children) {
+        if (!Format(child, scope_stream, sc, exe_ctx, addr, valobj,
+                    function_changed, initial_function))
+          return false;
+      }
+      return true;
+    };
+
+    for (auto &children : entry.children_stack) {
+      if (format_children(children)) {
+        s.Write(scope_stream.GetString().data(),
+                scope_stream.GetString().size());
+        return true;
+      }
     }
-    // Only if all items in a scope succeed, then do we print the output into
-    // the main stream
-    if (success)
-      s.Write(scope_stream.GetString().data(), scope_stream.GetString().size());
-  }
+
     return true; // Scopes always successfully print themselves
+  }
 
   case Entry::Type::Variable:
   case Entry::Type::VariableSynthetic:
@@ -2121,7 +2152,7 @@ static Status ParseInternal(llvm::StringRef &format, Entry &parent_entry,
                             uint32_t depth) {
   Status error;
   while (!format.empty() && error.Success()) {
-    const size_t non_special_chars = format.find_first_of("${}\\");
+    const size_t non_special_chars = format.find_first_of("${}\\|");
 
     if (non_special_chars == llvm::StringRef::npos) {
       // No special characters, just string bytes so add them and we are done
@@ -2158,6 +2189,14 @@ static Status ParseInternal(llvm::StringRef &format, Entry &parent_entry,
                 .drop_front(); // Skip the '}' as we are at the end of the scope
       return error;
 
+    case '|':
+      format = format.drop_front(); // Skip the '|'
+      if (parent_entry.type == Entry::Type::Scope)
+        parent_entry.StartAlternative();
+      else
+        parent_entry.AppendChar('|');
+      break;
+
     case '\\': {
       format = format.drop_front(); // Skip the '\' character
       if (format.empty()) {

lldb/unittests/Core/FormatEntityTest.cpp

@@ -8,6 +8,7 @@
 
 #include "lldb/Core/FormatEntity.h"
 #include "lldb/Utility/Status.h"
+#include "lldb/Utility/StreamString.h"
 
 #include "llvm/ADT/StringRef.h"
 #include "gtest/gtest.h"
@@ -160,3 +161,48 @@ TEST(FormatEntity, LookupAllEntriesInTree) {
         << "Formatting " << testString << " did not succeed";
   }
 }
+
+TEST(FormatEntity, ScopeAlt) {
+  StreamString stream;
+  FormatEntity::Entry format;
+  Status status = FormatEntity::Parse("{${frame.pc}|foo}", format);
+  ASSERT_TRUE(status.Success()) << status.AsCString();
+
+  FormatEntity::Format(format, stream, nullptr, nullptr, nullptr, nullptr,
+                       false, false);
+  EXPECT_EQ(stream.GetString(), "foo");
+}
+
+TEST(FormatEntity, EscapedPipeInScope) {
+  StreamString stream;
+  FormatEntity::Entry format;
+  Status status = FormatEntity::Parse("{foo\\|bar}", format);
+  ASSERT_TRUE(status.Success()) << status.AsCString();
+
+  FormatEntity::Format(format, stream, nullptr, nullptr, nullptr, nullptr,
+                       false, false);
+  EXPECT_EQ(stream.GetString(), "foo|bar");
+}
+
+TEST(FormatEntity, PipeOutsideScope) {
+  StreamString stream;
+  FormatEntity::Entry format;
+  Status status = FormatEntity::Parse("foo|bar", format);
+  ASSERT_TRUE(status.Success()) << status.AsCString();
+
+  FormatEntity::Format(format, stream, nullptr, nullptr, nullptr, nullptr,
+                       false, false);
+  EXPECT_EQ(stream.GetString(), "foo|bar");
+}
+
+TEST(FormatEntity, ScopeAltMultiple) {
+  StreamString stream;
+  FormatEntity::Entry format;
+  Status status =
+      FormatEntity::Parse("{${frame.pc}|${function.name}|foo}", format);
+  ASSERT_TRUE(status.Success()) << status.AsCString();
+
+  FormatEntity::Format(format, stream, nullptr, nullptr, nullptr, nullptr,
+                       false, false);
+  EXPECT_EQ(stream.GetString(), "foo");
+}