From cce63aca270c51fb3ce9790438e3c23864de0a87 Mon Sep 17 00:00:00 2001
From: Adrian Kummerlaender
Date: Wed, 23 Dec 2015 21:43:51 +0100
Subject: Implement file change tracking using `diff`

The newly introduced `ChangeTracker` class is now keeping track of all tracked file in addition to spawning and managing a corresponding `diff` instance that enables printing pretty _patch-style_ change summaries to the logging target.

This commit introduces `boost-process` and `diff` as dependencies of this library.
---
 src/change_tracker.cc | 94 +++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 94 insertions(+)
 create mode 100644 src/change_tracker.cc

(limited to 'src/change_tracker.cc')

diff --git a/src/change_tracker.cc b/src/change_tracker.cc
new file mode 100644
index 0000000..1373a8a
--- /dev/null
+++ b/src/change_tracker.cc
@@ -0,0 +1,94 @@
+#include "change_tracker.h"
+
+#include <sstream>
+
+#include <boost/optional.hpp>
+#include <boost/filesystem/fstream.hpp>
+
+namespace {
+
+// constants for increasing pair access readability
+constexpr unsigned int EMPLACE_SUCCESS = 1;
+constexpr unsigned int FILE_NAME       = 0;
+constexpr unsigned int DIFF_PROCESS    = 1;
+
+boost::process::context createContext() {
+	boost::process::context context;
+
+	context.environment     = boost::process::self::get_environment();
+	context.stdout_behavior = boost::process::capture_stream();
+	context.stdin_behavior  = boost::process::capture_stream();
+
+	return context;
+}
+
+}
+
+namespace utility {
+
+ChangeTracker::ChangeTracker(utility::Logger* logger):
+	logger_(logger),
+	children_() { }
+
+ChangeTracker::~ChangeTracker() {
+	for ( auto&& tracked : this->children_ ) {
+		std::get<DIFF_PROCESS>(tracked)->get_stdin().close();
+
+		this->logger_->forward(std::get<DIFF_PROCESS>(tracked)->get_stdout());
+
+		tracked.second->wait();
+	}
+}
+
+bool ChangeTracker::is_tracked(const std::string& file_path) const {
+	return this->children_.find(file_path) != this->children_.end();
+}
+
+// Begins tracking changes to a file reachable by a given path
+//
+// The actual tracking is performed by a `diff` instance that is
+// spawned by this method and managed by this class.
+// As `diff` is called as a new subprocess and because there is no
+// straight forward way for checking / enforcing if it has already
+// completed reading the initial contents of the file the command
+// is structured in the following sequence:
+//
+// diff -p - $file_path
+// 
+// This means that reading the final file contents is delegated to
+// `diff` while the initial file contents are read by this method
+// and written to `diff`'s standard input.
+//
+// If the `-` and `$file_path` arguments were exchanged there would
+// be no way to:
+//  - be sure the initial contents are read before the changing
+//    syscall that triggered this tracking in the first place is
+//    performed
+//  - be sure that the initial file contents are read completly
+//    as `diff` seemingly only reads the first block of the first
+//    file provided if the second file argument is standard input
+//
+bool ChangeTracker::track(const std::string& file_path) {
+	auto result = this->children_.emplace(
+		file_path,
+		std::make_unique<boost::process::child>(
+			boost::process::launch_shell("diff -p - " + file_name, createContext())
+		)
+	);
+
+	if ( std::get<EMPLACE_SUCCESS>(result) ) {
+		boost::filesystem::ifstream file(
+			std::get<FILE_NAME>(*result.first)
+		);
+
+		if ( file.is_open() ) {
+			std::get<DIFF_PROCESS>(*result.first)->get_stdin() << file.rdbuf();
+		}
+
+		return true;
+	} else {
+		return false;
+	}
+}
+
+}
-- 
cgit v1.2.3