Reimplement SignalSafeElfCache::Path

Summary: [Folly] Reimplement `SignalSafeElfCache::Path` minimally so that it meets the expectations of `std::basic_string` as used by the rest of `SignalSafeElfCache`.

Reviewed By: chadaustin, luciang

Differential Revision: D23152224

fbshipit-source-id: 48c7a7277d00a928d3de2937699ad112d6d560c2

folly/experimental/symbolizer/ElfCache.cpp

@@ -24,6 +24,16 @@
 namespace folly {
 namespace symbolizer {
 
+SignalSafeElfCache::Path::Path(
+    char const* const data,
+    std::size_t const size,
+    reentrant_allocator<char> const& alloc) noexcept
+    : data_{alloc} {
+  data_.reserve(size + 1);
+  data_.insert(data_.end(), data, data + size);
+  data_.insert(data_.end(), '\0');
+}
+
 std::shared_ptr<ElfFile> SignalSafeElfCache::getFile(StringPiece p) {
   struct cmp {
     bool operator()(Entry const& a, StringPiece b) const noexcept {

folly/experimental/symbolizer/ElfCache.h

@@ -21,6 +21,7 @@
 #include <mutex>
 #include <string>
 #include <unordered_map>
+#include <vector>
 
 #include <boost/intrusive/avl_set.hpp>
 
@@ -47,8 +48,42 @@ class SignalSafeElfCache : public ElfCacheBase {
  public:
   std::shared_ptr<ElfFile> getFile(StringPiece path) override;
 
-  using Path = std::
-      basic_string<char, std::char_traits<char>, reentrant_allocator<char>>;
+  //  Path
+  //
+  //  A minimal implementation of the subset of std::string used below, as if:
+  //
+  //    using Path = std::basic_string<
+  //        char, std::char_traits<char>, reentrant_allocator<char>>;
+  //
+  //  Since some library implementations of std::basic_string, as on CentOS 7,
+  //  do not build when instantiated with a non-default-constructible allocator,
+  //  and since other library replacements, such as folly::basic_fbstring, just
+  //  ignore the allocator parameter.
+  class Path {
+   public:
+    Path(
+        char const* data,
+        std::size_t size,
+        reentrant_allocator<char> const& alloc) noexcept;
+    Path() = delete;
+    Path(Path const&) = delete;
+    void operator=(Path const&) = delete;
+
+    /* implicit */ operator StringPiece() const noexcept {
+      return data_;
+    }
+
+    char const* c_str() const noexcept {
+      return data_.data();
+    }
+
+    friend bool operator<(Path const& a, Path const& b) noexcept {
+      return a.data_ < b.data_;
+    }
+
+   private:
+    std::vector<char, reentrant_allocator<char>> data_;
+  };
 
   struct Entry : boost::intrusive::avl_set_base_hook<> {
     Path path;