MINOR: [C++] Avoid multimap::find unspecified behavior (#48607)

### Rationale for this change

When a `std::multimap` has multiple entries with the same key, calling `m.find(key)` returns an unspecified element.

Historically, this returns the first matching element. However, this is not guaranteed, and recent libc++ changes make this return an arbitrary element. This can lead to surprising behavior, i.e. different values returned based on what other entries are in the multimap (and what the shape of the internal tree is).

### What changes are included in this PR?

Replace `m.find(key)` with `m.equal_range(key)` to preserve behavior and have predictable results. The behavior of this is guaranteed to return a range of all matching elements in insertion order, and the beginning of the range is the same element as what's normally returned by `m.find(key)`.

### Are these changes tested?

Yes

### Are there any user-facing changes?

No

Authored-by: Jordan Rupprecht <[email protected]>
Signed-off-by: David Li <[email protected]>

Author: rupprecht

cpp/src/arrow/flight/transport/grpc/grpc_client.cc

@@ -25,6 +25,7 @@
 #include <sstream>
 #include <string>
 #include <thread>
+#include <tuple>
 #include <unordered_map>
 #include <utility>
 
@@ -448,10 +449,11 @@ arrow::Result<std::pair<std::string, std::string>> GetBearerTokenHeader(
   // Get the auth token if it exists, this can be in the initial or the trailing metadata.
   auto trailing_headers = context.GetServerTrailingMetadata();
   auto initial_headers = context.GetServerInitialMetadata();
-  auto bearer_iter = trailing_headers.find(internal::kAuthHeader);
-  if (bearer_iter == trailing_headers.end()) {
-    bearer_iter = initial_headers.find(internal::kAuthHeader);
-    if (bearer_iter == initial_headers.end()) {
+  auto [bearer_iter, bearer_end] = trailing_headers.equal_range(internal::kAuthHeader);
+  if (bearer_iter == bearer_end) {
+    std::tie(bearer_iter, bearer_end) =
+        initial_headers.equal_range(internal::kAuthHeader);
+    if (bearer_iter == bearer_end) {
       return std::make_pair("", "");
     }
   }

cpp/src/arrow/flight/transport/grpc/grpc_server.cc

@@ -307,9 +307,10 @@ class GrpcServiceHandler final : public FlightService::Service {
       }
     } else {
       const auto client_metadata = context->client_metadata();
-      const auto auth_header = client_metadata.find(kGrpcAuthHeader);
+      const auto [auth_header, auth_header_end] =
+          client_metadata.equal_range(kGrpcAuthHeader);
       std::string token;
-      if (auth_header == client_metadata.end()) {
+      if (auth_header == auth_header_end) {
         token = "";
       } else {
         token = std::string(auth_header->second.data(), auth_header->second.length());

cpp/src/arrow/flight/transport/grpc/util_internal.cc

@@ -55,31 +55,25 @@ static bool FromGrpcContext(const ::grpc::ClientContext& ctx,
   const std::multimap<::grpc::string_ref, ::grpc::string_ref>& trailers =
       ctx.GetServerTrailingMetadata();
 
-  const auto code_val = trailers.find(kGrpcStatusCodeHeader);
-  if (code_val == trailers.end()) return false;
+  const auto [code_val_begin, code_val_end] = trailers.equal_range(kGrpcStatusCodeHeader);
+  if (code_val_begin == code_val_end) return false;
 
-  const auto message_val = trailers.find(kGrpcStatusMessageHeader);
-  const std::optional<std::string> message =
-      message_val == trailers.end()
-          ? std::nullopt
-          : std::optional<std::string>(
-                std::string(message_val->second.data(), message_val->second.size()));
+  std::optional<std::string> message;
+  if (const auto [it, end] = trailers.equal_range(kGrpcStatusMessageHeader); it != end) {
+    message = std::string(it->second.data(), it->second.size());
+  }
 
-  const auto detail_val = trailers.find(kGrpcStatusDetailHeader);
-  const std::optional<std::string> detail_message =
-      detail_val == trailers.end()
-          ? std::nullopt
-          : std::optional<std::string>(
-                std::string(detail_val->second.data(), detail_val->second.size()));
+  std::optional<std::string> detail_message;
+  if (const auto [it, end] = trailers.equal_range(kGrpcStatusDetailHeader); it != end) {
+    detail_message = std::string(it->second.data(), it->second.size());
+  }
 
-  const auto grpc_detail_val = trailers.find(kBinaryErrorDetailsKey);
-  const std::optional<std::string> detail_bin =
-      grpc_detail_val == trailers.end()
-          ? std::nullopt
-          : std::optional<std::string>(std::string(grpc_detail_val->second.data(),
-                                                   grpc_detail_val->second.size()));
+  std::optional<std::string> detail_bin;
+  if (const auto [it, end] = trailers.equal_range(kBinaryErrorDetailsKey); it != end) {
+    detail_bin = std::string(it->second.data(), it->second.size());
+  }
 
-  std::string code_str(code_val->second.data(), code_val->second.size());
+  std::string code_str(code_val_begin->second.data(), code_val_begin->second.size());
   *status = internal::ReconstructStatus(code_str, current_status, std::move(message),
                                         std::move(detail_message), std::move(detail_bin),
                                         std::move(flight_status_detail));