Every slog line the app wrote on Android went to /dev/null, including
the one naming the error it was about to os.Exit on. #52 is what that
cost: a process that vanished with no tombstone, no AndroidRuntime
stack and nothing in `logcat -b crash`, at Priority/Critical for
months, whose entire diagnosis was one sLogger.Error main.go was
already writing.
backend/androidlog is a slog.Handler over __android_log_write, chosen
in main() by build tag rather than by a runtime check so that a desktop
binary links no cgo for a platform it cannot run on.
**Everything except the write itself is untagged.** That is
androidpayload.go's discipline pushed as far as it goes: the only
toolchain that compiles the android tag is a cross-compiler and the
only thing that runs it is a phone, so the priority mapping, the
formatting, the chunking and the handler's own attr and group
bookkeeping are ordinary Go that `go test` exercises everywhere, and
android.go is fifteen lines that hand a string to liblog.
Four things in it are load-bearing.
**The tag is a fixed string, not the application id.** The debug build
carries `applicationIdSuffix ".dev"` so it can be installed beside the
release app, and it is the only build whose WebView can be inspected --
so a tag derived from the id is a different tag on the one build
anybody debugging this app is running, and the filter meant to show
these lines would hide them exactly where they were being looked for.
**The priorities are android/log.h's own values, asserted twice.**
android.go carries constant expressions that do not compile as uint if
the header renumbers; the untagged test writes the six numbers out
longhand, because comparing a constant to itself passes on any
renumbering. A wrong priority is the failure that hides rather than
breaks -- logcat prints whatever number it is handed, so an Error filed
as Info is present, correct, and invisible to every filter.
**Formatting is delegated to slog's TextHandler.** WithAttrs and
WithGroup are the half of slog.Handler that is easy to get subtly
wrong, and a logger whose groups are wrong is a logger nobody reads.
The derived handlers share the parent's buffer *and its mutex*: a
second mutex would guard nothing, and two loggers derived from one
would splice their bytes into a single line under load.
**A line is chunked, because liblog drops what does not fit.** The
kernel logger's entry is 4068 bytes for tag and message together and
the remainder goes without comment, so a long record would be truncated
in the middle of the thing worth reading.
Time and level are dropped from the formatted line, since logcat stamps
every entry with both -- and dropping them by *key* also ate a caller's
own "level" attribute, which the on-device probe caught and
TestACallersOwnLevelAttrSurvives now holds. ReplaceAttr sees an empty
group path for the built-ins and for every top-level attribute alike,
so the kinds are what separate them.
Verified on the reference device (TLP301, Android 14): a debug build
logs I/W/E under the `yellowjacket` tag at the right priorities, and
the first thing it surfaced was a real warning nobody could previously
see -- `champion index rebuild failed ... disk I/O error (6410)`.
Closes#160
logan
168e588387