implement the fopencookie extension to stdio

notes added by maintainer:

this function is a GNU extension. it was chosen over the similar BSD
function funopen because the latter depends on fpos_t being an
arithmetic type as part of its public API, conflicting with our
definition of fpos_t and with the intent that it be an opaque type. it
was accepted for inclusion because, despite not being widely used, it
is usually very difficult to extricate software using it from the
dependency on it.

calling pattern for the read and write callbacks is not likely to
match glibc or other implementations, but should work with any
reasonable callbacks. in particular the read function is never called
without at least one byte being needed to satisfy its caller, so that
spurious blocking is not introduced.

contracts for what callbacks called from inside libc/stdio can do are
always complicated, and at some point still need to be specified
explicitly. at the very least, the callbacks must return or block
indefinitely (they cannot perform nonlocal exits) and they should not
make calls to stdio using their own FILE as an argument.

include/stdio.h

@@ -182,6 +182,20 @@ int vasprintf(char **, const char *, __isoc_va_list);
 #ifdef _GNU_SOURCE
 char *fgets_unlocked(char *, int, FILE *);
 int fputs_unlocked(const char *, FILE *);
+
+typedef ssize_t (cookie_read_function_t)(void *, char *, size_t);
+typedef ssize_t (cookie_write_function_t)(void *, const char *, size_t);
+typedef int (cookie_seek_function_t)(void *, off_t *, int);
+typedef int (cookie_close_function_t)(void *);
+
+typedef struct {
+	cookie_read_function_t *read;
+	cookie_write_function_t *write;
+	cookie_seek_function_t *seek;
+	cookie_close_function_t *close;
+} cookie_io_functions_t;
+
+FILE *fopencookie(void *, const char *, cookie_io_functions_t);
 #endif
 
 #if defined(_LARGEFILE64_SOURCE) || defined(_GNU_SOURCE)

src/stdio/fopencookie.c

@@ -0,0 +1,138 @@
+#define _GNU_SOURCE
+#include "stdio_impl.h"
+#include <stdlib.h>
+#include <sys/ioctl.h>
+#include <fcntl.h>
+#include <errno.h>
+#include <string.h>
+
+struct fcookie {
+	void *cookie;
+	cookie_io_functions_t iofuncs;
+};
+
+struct cookie_FILE {
+	FILE f;
+	struct fcookie fc;
+	unsigned char buf[UNGET+BUFSIZ];
+};
+
+static size_t cookieread(FILE *f, unsigned char *buf, size_t len)
+{
+	struct fcookie *fc = f->cookie;
+	ssize_t ret = -1;
+	size_t remain = len, readlen = 0;
+	size_t len2 = len - !!f->buf_size;
+
+	if (!fc->iofuncs.read) goto bail;
+
+	if (len2) {
+		ret = fc->iofuncs.read(fc->cookie, (char *) buf, len2);
+		if (ret <= 0) goto bail;
+
+		readlen += ret;
+		remain -= ret;
+	}
+
+	if (!f->buf_size || remain > !!f->buf_size) return readlen;
+
+	f->rpos = f->buf;
+	ret = fc->iofuncs.read(fc->cookie, (char *) f->rpos, f->buf_size);
+	if (ret <= 0) goto bail;
+	f->rend = f->rpos + ret;
+
+	buf[readlen++] = *f->rpos++;
+
+	return readlen;
+
+bail:
+	f->flags |= ret == 0 ? F_EOF : F_ERR;
+	f->rpos = f->rend = f->buf;
+	return readlen;
+}
+
+static size_t cookiewrite(FILE *f, const unsigned char *buf, size_t len)
+{
+	struct fcookie *fc = f->cookie;
+	ssize_t ret;
+	size_t len2 = f->wpos - f->wbase;
+	if (!fc->iofuncs.write) return len;
+	if (len2) {
+		f->wpos = f->wbase;
+		if (cookiewrite(f, f->wpos, len2) < len2) return 0;
+	}
+	ret = fc->iofuncs.write(fc->cookie, (const char *) buf, len);
+	if (ret < 0) {
+		f->wpos = f->wbase = f->wend = 0;
+		f->flags |= F_ERR;
+		return 0;
+	}
+	return ret;
+}
+
+static off_t cookieseek(FILE *f, off_t off, int whence)
+{
+	struct fcookie *fc = f->cookie;
+	int res;
+	if (whence > 2U) {
+		errno = EINVAL;
+		return -1;
+	}
+	if (!fc->iofuncs.seek) {
+		errno = ENOTSUP;
+		return -1;
+	}
+	res = fc->iofuncs.seek(fc->cookie, &off, whence);
+	if (res < 0)
+		return res;
+	return off;
+}
+
+static int cookieclose(FILE *f)
+{
+	struct fcookie *fc = f->cookie;
+	if (fc->iofuncs.close) return fc->iofuncs.close(fc->cookie);
+	return 0;
+}
+
+FILE *fopencookie(void *cookie, const char *mode, cookie_io_functions_t iofuncs)
+{
+	struct cookie_FILE *f;
+
+	/* Check for valid initial mode character */
+	if (!strchr("rwa", *mode)) {
+		errno = EINVAL;
+		return 0;
+	}
+
+	/* Allocate FILE+fcookie+buffer or fail */
+	if (!(f=malloc(sizeof *f))) return 0;
+
+	/* Zero-fill only the struct, not the buffer */
+	memset(&f->f, 0, sizeof f->f);
+
+	/* Impose mode restrictions */
+	if (!strchr(mode, '+')) f->f.flags = (*mode == 'r') ? F_NOWR : F_NORD;
+
+	/* Set up our fcookie */
+	f->fc.cookie = cookie;
+	f->fc.iofuncs.read = iofuncs.read;
+	f->fc.iofuncs.write = iofuncs.write;
+	f->fc.iofuncs.seek = iofuncs.seek;
+	f->fc.iofuncs.close = iofuncs.close;
+
+	f->f.fd = -1;
+	f->f.cookie = &f->fc;
+	f->f.buf = f->buf + UNGET;
+	f->f.buf_size = BUFSIZ;
+	f->f.lbf = EOF;
+
+	/* Initialize op ptrs. No problem if some are unneeded. */
+	f->f.read = cookieread;
+	f->f.write = cookiewrite;
+	f->f.seek = cookieseek;
+	f->f.close = cookieclose;
+
+	/* Add new FILE to open file list */
+	return __ofl_add(&f->f);
+}