[BUG] Make file_http_server.h compile on Windows

CHANGELOG.md

@@ -22,6 +22,9 @@ Increment the:
 * [CODE HEALTH] Move remaining API test helpers into anonymous namespaces
   [#4301](https://github.com/open-telemetry/opentelemetry-cpp/pull/4301)
 
+* [BUG] Make file_http_server.h compile on Windows
+  [#4300](https://github.com/open-telemetry/opentelemetry-cpp/pull/4300)
+
 * [CODE HEALTH] Move metrics storage test fixtures into anonymous namespace
   [#4286](https://github.com/open-telemetry/opentelemetry-cpp/pull/4286)
 

ext/include/opentelemetry/ext/http/server/file_http_server.h

@@ -4,10 +4,13 @@
 #pragma once
 
 #include <fstream>
-#include <iostream>
+#include <sstream>
 #include <string>
 #include <unordered_map>
 #include <vector>
+#ifdef _WIN32
+#  include <algorithm>  // std::replace, used only in the _WIN32 path below
+#endif
 
 #include "opentelemetry/ext/http/server/http_server.h"
 
@@ -28,7 +31,7 @@ class FileHttpServer : public HTTP_SERVER_NS::HttpServer
     os << host << ":" << port;
     setServerName(os.str());
     addListeningPort(port);
-  };
+  }
 
   /**
    * Set the HTTP server to serve static files from the root of host:port.
@@ -52,10 +55,15 @@ class FileHttpServer : public HTTP_SERVER_NS::HttpServer
   bool FileGetSuccess(const std::string &filename, std::vector<char> &result)
   {
 #ifdef _WIN32
-    std::replace(filename.begin(), filename.end(), '/', '\\');
+    // std::replace writes through its iterators, so it needs a modifiable string rather than
+    // the const parameter.
+    std::string path = filename;
+    std::replace(path.begin(), path.end(), '/', '\\');
+#else
+    const std::string &path = filename;
 #endif
     std::streampos size;
-    std::ifstream file(filename, std::ios::in | std::ios::binary | std::ios::ate);
+    std::ifstream file(path, std::ios::in | std::ios::binary | std::ios::ate);
     if (file.is_open())
     {
       size = file.tellg();
@@ -69,7 +77,7 @@ class FileHttpServer : public HTTP_SERVER_NS::HttpServer
       return true;
     }
     return false;
-  };
+  }
 
   /**
    * Returns the extension of a file
@@ -78,10 +86,10 @@ class FileHttpServer : public HTTP_SERVER_NS::HttpServer
    */
   std::string GetMimeContentType(const std::string &filename)
   {
-    std::string file_ext = filename.substr(filename.find_last_of(".") + 1);
+    std::string file_ext = filename.substr(filename.find_last_of('.') + 1);
     auto file_type       = mime_types_.find(file_ext);
     return (file_type != mime_types_.end()) ? file_type->second : HTTP_SERVER_NS::CONTENT_TYPE_TEXT;
-  };
+  }
 
   /**
    * Returns the standardized name of a file by removing backslashes, and
@@ -97,7 +105,7 @@ class FileHttpServer : public HTTP_SERVER_NS::HttpServer
     }
     // If filename appears to be a directory, serve the hypothetical index.html
     // file there
-    if (name.find(".") == std::string::npos)
+    if (name.find('.') == std::string::npos)
       name += "/index.html";
 
     return name;

ext/test/http/BUILD

@@ -1,8 +1,21 @@
 # Copyright The OpenTelemetry Authors
 # SPDX-License-Identifier: Apache-2.0
 
+load("@rules_cc//cc:cc_library.bzl", "cc_library")
 load("@rules_cc//cc:cc_test.bzl", "cc_test")
 
+# file_http_server.h is an installed header no other target includes, so it was never compiled.
+# This library compiles it (the POSIX branch here, the Windows branch on the Windows runners) so
+# a build regression in it is caught.
+cc_library(
+    name = "file_http_server_header_compile",
+    srcs = ["file_http_server_header_compile.cc"],
+    tags = ["test"],
+    deps = [
+        "//ext:headers",
+    ],
+)
+
 cc_test(
     name = "curl_http_test",
     srcs = [

ext/test/http/CMakeLists.txt

@@ -22,3 +22,11 @@ gtest_add_tests(
   TARGET ${URL_PARSER_FILENAME}
   TEST_PREFIX ext.http.urlparser.
   TEST_LIST ${URL_PARSER_FILENAME})
+
+# file_http_server.h is an installed header that no other target includes, so it
+# was never compiled. This object library compiles it (the POSIX branch here,
+# the Windows branch on the Windows runners) so a build regression in it is
+# caught. It is not run.
+add_library(file_http_server_header_compile OBJECT
+            file_http_server_header_compile.cc)
+target_link_libraries(file_http_server_header_compile PRIVATE opentelemetry_ext)

ext/test/http/file_http_server_header_compile.cc

@@ -0,0 +1,9 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+// file_http_server.h is an installed public header that no other translation unit in the
+// repository includes, so nothing compiled it and a build error could sit there unnoticed (the
+// _WIN32 branch did exactly that). This translation unit exists only to compile the header, so
+// CI builds the POSIX branch here and the Windows branch on the Windows runners. It has no
+// main() and is never run.
+#include "opentelemetry/ext/http/server/file_http_server.h"  // IWYU pragma: keep