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* Output destination (§5.1 parity rows "Test-time silencing, capture and
* assertion" and "Multiple sinks in parallel"; PRD R18).
*
* Swapping the writer is how a test silences output and how a later sink is
* added — the logger never branches on the destination. Until a writer is
* installed the destination is stdout, so zero-configuration usage emits with
* no setup at all; installing `null` silences output explicitly.
*
* A destination is handed the rendered line *and* the record it was rendered
* from. The line is what a terminal or a file wants; the record is what a
* structured sink needs, because a record cannot be recovered from a
* human-rendered line and `human` is the emitter's default format — a sink that
* had only the line would be inert unless the caller also switched the format
* (see `service/transport.ts`). The second argument is optional, and a
* line-only destination simply ignores it, so every existing writer is
* unchanged.
*/
import type {LogRecord} from './record.ts';
export interface Writer {
write(line: string, record?: LogRecord): void;
}
export const stdoutWriter: Writer = {
write(line: string): void {
process.stdout.write(line);
},
};
export const stderrWriter: Writer = {
write(line: string): void {
process.stderr.write(line);
},
};
/**
* `undefined` means "never installed": the destination defaults to stdout so
* zero-configuration usage works. `null` is an explicit choice to silence, and
* must not fall back to stdout.
*/
let active: Writer | null | undefined = undefined;
/** Replace the destination. `null` silences output. */
export function setWriter(writer: Writer | null): void {
active = writer;
}
/** The current destination, or `null` when silenced. */
export function getWriter(): Writer | null {
return active === undefined ? stdoutWriter : active;
}
/**
* Write the same line (and the same record) to every destination, in order, and
* let no single destination break the others or the caller.
*
* This is R18's "a logging call never waits on, or fails because of, a sink"
* extended to N sinks: each write is isolated, so a sink that throws — a full
* disk, a closed stream, a broken transport — costs its own copy of the line
* and nothing else. The failure is swallowed rather than reported here; a sink
* that can say something useful about a failed delivery reports it itself (see
* `service/transport.ts`, ruling D3).
*/
export function createFanoutWriter(writers: readonly Writer[]): Writer {
return {
write(line: string, record?: LogRecord): void {
for (const writer of writers) {
try {
writer.write(line, record);
} catch {
// Swallowed by design: one destination must not suppress the
// rest of the fan-out, and must not escape into the log call.
}
}
},
};
}
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