Handle resuming into same subdir

Before this change, when resuming into the same sudir that we resumed
from, the older data files would be overwritten, because we do not track
the writer's file indices, and resuming for some writers would be very
non-trivial (metadata handling etc).

To avoid data loss, following the discussion around issue #56 on GitHub,
we enact the following policy:

1. nothing changes if we resume into a new directory;
2. we prevent resuming into an existing directory that is _not_ the same
   we are resuming from;
3. when resuming into the same directory we resume from, the actual
   problem dir is shifted into the first available `resumeN` subdir.

src/ProblemCore.cc

@@ -992,6 +992,16 @@ static int mkdir_p(string const& path, mode_t mode)
 	return err == 0 ? 0 : errno;
 }
 
+static string canonical_path(string const& path)
+{
+	char *canonical_c = realpath(path.c_str(), NULL);
+	string canonical(canonical_c);
+	free(canonical_c);
+	return canonical;
+}
+
+constexpr mode_t problem_dir_mode = S_IRWXU | S_IRWXG | S_IRWXO;
+
 string const&
 ProblemCore::create_problem_dir(void)
 {
@@ -1006,13 +1016,63 @@ ProblemCore::create_problem_dir(void)
 		time_str[17] = '\0';
 		m_problem_dir = "./tests/" + m_name + string(time_str);
 	}
+
+	// ensure the problem dir does _not_ have a / at the end
+	while (m_problem_dir.back() == '/')
+		m_problem_dir = m_problem_dir.substr(0, m_problem_dir.length() - 1);
 	cout << "Using problem dir " << m_problem_dir << endl;
 
 	// TODO it should be possible to specify a directory with %-like
 	// replaceable strings, such as %{problem} => problem name,
 	// %{time} => launch time, etc.
 
-	int err = mkdir_p(m_problem_dir.c_str(), S_IRWXU | S_IRWXG | S_IRWXO);
+	int err = mkdir_p(m_problem_dir.c_str(), problem_dir_mode);
+
+	// If the directory exists, and we are resuming, then
+	// 1. we only continue if we're resuming into the same directory we're resuming from
+	// 2. we resume into a subdirectory, to avoid overwriting the original data files
+	if (err == EEXIST && gdata->resume) {
+		// we want to check that the canonical form of the problem is the
+		// initial substring of the canonical form of the resume file, including the path
+		// separator (i.e. don't mess if we are resuming into tests/something from
+		// tests/something-else/data/hotfile). The canonical path doesn't have a terminating /,
+		// so we add one.
+		const string& resume_file = gdata->clOptions->resume_fname;
+		string canonical_problem_dir( canonical_path(m_problem_dir) + "/");
+		string canonical_resume_file( canonical_path(resume_file) );
+		if (canonical_resume_file.substr(0, canonical_problem_dir.length()) != canonical_problem_dir) {
+			throw runtime_error("refusing to resume from " +
+				resume_file + " (" + canonical_resume_file + ") into unrelated, existing " +
+				m_problem_dir + " (" + canonical_problem_dir + ")");
+		}
+
+		// OK, we're now in the ’resume in the same directory’ case. To avoid overwriting the previous
+		// simulation data, and since we cannot (currently) correctly recover the index and restart,
+		// we actually shift our problem dir into problemdir/resumeN, where N is the first available index
+		// (so that e.g. the third time we resume, our effective problem dir becomes
+		// problemdir/resume003/
+		// TODO FIXME of course it would actually be preferrable to resume correctly instead
+		unsigned resume_count = 0;
+		char new_dir[] = { '/', 'r', 'e', 's', 'u', 'm', 'e', '0', '0', '0', 0 };
+		string candidate;
+		cout << "Resuming into same directory, looking for next candidate" << endl;
+#define MAX_RESUMES 1000
+		for ( ; resume_count < MAX_RESUMES; ++resume_count) {
+			snprintf(new_dir + 7, 4, "%03u", resume_count);
+			candidate = m_problem_dir + new_dir;
+			err = mkdir_p(candidate.c_str(), problem_dir_mode);
+			if (err != EEXIST) break;
+			cout << "\t" << candidate << " exists" << endl;
+		}
+		if (resume_count == MAX_RESUMES) {
+			throw runtime_error("Tried to resume too much (1,000 attempts made), please look into this");
+		}
+		m_problem_dir = candidate;
+		if (err == 0)
+			cout << "OK, resuming into " + candidate << endl;
+		// if err != 0, this will be caught by the next switch, shared with the non-resume case
+	}
+
 	switch (err) {
 		case EEXIST:
 			cerr << "WARNING: problem directory " << m_problem_dir << " exists already, overwriting." << endl;