Write a timestamped transcript per run

A run whose result nobody recorded is a run that has to be repeated on a
human's finger, and this daemon printed to stdout and nowhere else. Two real
authentication runs -- an enrolled finger and a wrong-finger control -- were
performed and their output is simply gone.

A single shared log path would be worse than none: the next run, including a
quick control, overwrites the interesting one. That very nearly cost this
project the transcript of its first successful authentication, which is why
the harness moved to per-run files.

Teeing is done at the file-descriptor level rather than by wrapping a stream.
std::println writes to stdout through C stdio, so an ostream wrapper captures
nothing; routing fd 1 through tee catches every line, including whatever the
library prints.

implementations/main.cpp

@@ -56,6 +56,7 @@ bool g_listeners = false;
 bool g_auth = false;
 int g_frames = 40;
 int g_frameGapMs = 500;
+std::string g_logDir = "/var/log/fingerprintd";
 std::uint32_t g_gid = 0;
 std::string g_taPath = "/lib/firmware/focal64.mbn";
 std::string g_cfgPath = "/lib/firmware/fingerprintd.json";
@@ -66,6 +67,30 @@ qcomtee_object* g_root = QCOMTEE_OBJECT_NULL;
 // around it so the supplicant thread can be stopped while blocked in the
 // kernel waiting for QTEE.
 //
+// Every run writes its own timestamped transcript. Not a convenience: a run
+// whose result nobody recorded is a run that has to be repeated on a human's
+// finger. And a SINGLE shared log path is worse than none -- the next run,
+// including a quick control, destroys the interesting one, which is how the
+// first successful authentication in this project was very nearly lost.
+//
+// Done at the file-descriptor level rather than by wrapping a stream, because
+// std::println writes to stdout through C stdio: an ostream wrapper would
+// capture nothing. Routing fd 1 through tee catches every line including the
+// ones libqcomtee prints.
+bool StartTranscript(const std::string& dir) {
+    std::error_code ec;
+    std::filesystem::create_directories(dir, ec);
+    auto now = std::chrono::system_clock::now();
+    std::string path = std::format("{}/{:%Y%m%d-%H%M%S}.log", dir,
+                                   std::chrono::floor<std::chrono::seconds>(now));
+    FILE* t = ::popen(std::format("tee {}", path).c_str(), "w");
+    if (!t) return false;
+    ::dup2(::fileno(t), 1);
+    ::setvbuf(stdout, nullptr, _IOLBF, 0);
+    std::println("transcript: {}", path);
+    return true;
+}
+
 // tee_call_t's second parameter is `unsigned long` on glibc and `int` on musl
 // (qcomtee_object.h keys it off __GLIBC__), so the signature has to match or
 // the function pointer will not convert. The native build is glibc and the
@@ -1136,12 +1161,15 @@ int main(int argc, char** argv) {
         if (a == "--rpmb-write") g_rpmbWrite = true;
         if (a == "--auth") { g_auth = true; g_listeners = true; }
         if (a.starts_with("--frames=")) g_frames = std::stoi(std::string(a.substr(9)));
+        if (a.starts_with("--log-dir=")) g_logDir = a.substr(10);
         if (a.starts_with("--sfs-root=")) g_sfsRoot = a.substr(11);
         if (a.starts_with("--gid=")) g_gid = static_cast<std::uint32_t>(
             std::stoul(std::string(a.substr(6))));
     }
-    if (probe)
+    if (probe) {
+        StartTranscript(g_logDir);
         return Probe();
+    }
 
     std::println(std::cerr,
                  "fingerprintd {}: no runtime yet. --probe-tee reaches QTEE; "