libbb: introduce and use nonblock_safe_read(). Yay!

Our shells are immune from this nasty O_NONBLOCK now!

function                                             old     new   delta
nonblock_safe_read                                     -      78     +78
file_get                                             276     295     +19
generateMTFValues                                    428     435      +7
read_line_input                                     1776    1772      -4
preadbuffer                                          543     450     -93
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 2/2 up/down: 104/-97)             Total: 7 bytes
   text    data     bss     dec     hex filename
 615190     715   23924  639829   9c355 busybox_old
 615168     715   23924  639807   9c33f busybox_unstripped

6 files changed, 75 insertions, 15 deletions

diff --git a/include/libbb.h b/include/libbb.h
index a91eac466..eb8b2f620 100644
--- a/include/libbb.h
+++ b/include/libbb.h
@@ -475,6 +475,7 @@ extern void *xzalloc(size_t size);
 extern void *xrealloc(void *old, size_t size);
 
 extern ssize_t safe_read(int fd, void *buf, size_t count);
+extern ssize_t nonblock_safe_read(int fd, void *buf, size_t count);
 extern ssize_t full_read(int fd, void *buf, size_t count);
 extern void xread(int fd, void *buf, size_t count);
 extern unsigned char xread_char(int fd);
@@ -482,6 +483,7 @@ extern unsigned char xread_char(int fd);
 extern char *reads(int fd, char *buf, size_t count);
 // Read one line a-la fgets. Reads byte-by-byte.
 // Useful when it is important to not read ahead.
+// Bytes are appended to pfx (which must be malloced, or NULL).
 extern char *xmalloc_reads(int fd, char *pfx);
 extern ssize_t read_close(int fd, void *buf, size_t count);
 extern ssize_t open_read_close(const char *filename, void *buf, size_t count);
diff --git a/libbb/lineedit.c b/libbb/lineedit.c
index 529344f6c..9aab63702 100644
--- a/libbb/lineedit.c
+++ b/libbb/lineedit.c
@@ -1408,9 +1408,9 @@ int read_line_input(const char *prompt, char *command, int maxsize, line_input_t
 	parse_and_put_prompt(prompt);
 
 	while (1) {
-		fflush(stdout);
+		fflush(NULL);
 
-		if (safe_read(STDIN_FILENO, &c, 1) < 1) {
+		if (nonblock_safe_read(STDIN_FILENO, &c, 1) < 1) {
 			/* if we can't read input then exit */
 			goto prepare_to_die;
 		}
diff --git a/libbb/read.c b/libbb/read.c
index 502d407c4..2cd86b81b 100644
--- a/libbb/read.c
+++ b/libbb/read.c
@@ -20,6 +20,58 @@ ssize_t safe_read(int fd, void *buf, size_t count)
 	return n;
 }
 
+/* Suppose that you are a shell. You start child processes.
+ * They work and eventually exit. You want to get user input.
+ * You read stdin. But what happens if last child switched
+ * its stdin into O_NONBLOCK mode?
+ *
+ * *** SURPRISE! It will affect the parent too! ***
+ * *** BIG SURPRISE! It stays even after child exits! ***
+ *
+ * This is a design bug in UNIX API.
+ *      fcntl(0, F_SETFL, fcntl(0, F_GETFL, 0) | O_NONBLOCK);
+ * will set nonblocking mode not only on _your_ stdin, but
+ * also on stdin of your parent, etc.
+ *
+ * In general,
+ *      fd2 = dup(fd1);
+ *      fcntl(fd2, F_SETFL, fcntl(fd2, F_GETFL, 0) | O_NONBLOCK);
+ * sets both fd1 and fd2 to O_NONBLOCK. This includes cases
+ * where duping is done implicitly by fork() etc.
+ *
+ * We need
+ *      fcntl(fd2, F_SETFD, fcntl(fd2, F_GETFD, 0) | O_NONBLOCK);
+ * (note SETFD, not SETFL!) but such thing doesn't exist.
+ *
+ * Alternatively, we need nonblocking_read(fd, ...) which doesn't
+ * require O_NONBLOCK dance at all. Actually, it exists:
+ *      n = recv(fd, buf, len, MSG_DONTWAIT);
+ *      "MSG_DONTWAIT:
+ *      Enables non-blocking operation; if the operation
+ *      would block, EAGAIN is returned."
+ * but recv() works only for sockets!
+ *
+ * So far I don't see any good solution, I can only propose
+ * that affected readers should be careful and use this routine,
+ * which detects EAGAIN and uses poll() to wait on the fd.
+ * Thanksfully, poll() doesn't give rat's ass about O_NONBLOCK flag.
+ */
+ssize_t nonblock_safe_read(int fd, void *buf, size_t count)
+{
+	struct pollfd pfd[1];
+	ssize_t n;
+
+	while (1) {
+		n = safe_read(fd, buf, count);
+		if (n >= 0 || errno != EAGAIN)
+			return n;
+		/* fd is in O_NONBLOCK mode. Wait using poll and repeat */
+		pfd[0].fd = fd;
+		pfd[0].events = POLLIN;
+		safe_poll(pfd, 1, -1);
+	}
+}
+
 /*
  * Read all of the supplied buffer from a file.
  * This does multiple reads as necessary.
@@ -93,6 +145,7 @@ char *reads(int fd, char *buffer, size_t size)
 
 // Read one line a-la fgets. Reads byte-by-byte.
 // Useful when it is important to not read ahead.
+// Bytes are appended to pfx (which must be malloced, or NULL).
 char *xmalloc_reads(int fd, char *buf)
 {
 	char *p;
@@ -106,7 +159,8 @@ char *xmalloc_reads(int fd, char *buf)
 			p = buf + sz;
 			sz += 128;
 		}
-		if (safe_read(fd, p, 1) != 1) { /* EOF/error */
+		/* nonblock_safe_read() because we are used by e.g. shells */
+		if (nonblock_safe_read(fd, p, 1) != 1) { /* EOF/error */
 			if (p == buf) {
 				/* we read nothing [and buf was NULL initially] */
 				free(buf);
diff --git a/shell/ash.c b/shell/ash.c
index 65f94f682..debe8ecdd 100644
--- a/shell/ash.c
+++ b/shell/ash.c
@@ -5428,7 +5428,7 @@ expbackq(union node *cmd, int quoted, int quotes)
  read:
 		if (in.fd < 0)
 			break;
-		i = safe_read(in.fd, buf, sizeof(buf));
+		i = nonblock_safe_read(in.fd, buf, sizeof(buf));
 		TRACE(("expbackq: read returns %d\n", i));
 		if (i <= 0)
 			break;
@@ -8678,7 +8678,7 @@ preadfd(void)
  retry:
 #if ENABLE_FEATURE_EDITING
 	if (!iflag || parsefile->fd)
-		nr = safe_read(parsefile->fd, buf, BUFSIZ - 1);
+		nr = nonblock_safe_read(parsefile->fd, buf, BUFSIZ - 1);
 	else {
 #if ENABLE_FEATURE_TAB_COMPLETION
 		line_input_state->path_lookup = pathval();
@@ -8700,9 +8700,11 @@ preadfd(void)
 		}
 	}
 #else
-	nr = safe_read(parsefile->fd, buf, BUFSIZ - 1);
+	nr = nonblock_safe_read(parsefile->fd, buf, BUFSIZ - 1);
 #endif
 
+#if 0
+/* nonblock_safe_read() handles this problem */
 	if (nr < 0) {
 		if (parsefile->fd == 0 && errno == EWOULDBLOCK) {
 			int flags = fcntl(0, F_GETFL);
@@ -8715,6 +8717,7 @@ preadfd(void)
 			}
 		}
 	}
+#endif
 	return nr;
 }
 
@@ -11801,7 +11804,7 @@ readcmd(int argc, char **argv)
 	backslash = 0;
 	STARTSTACKSTR(p);
 	do {
-		if (read(0, &c, 1) != 1) {
+		if (nonblock_safe_read(0, &c, 1) != 1) {
 			status = 1;
 			break;
 		}
diff --git a/shell/hush.c b/shell/hush.c
index 4d4843173..820fd888d 100644
--- a/shell/hush.c
+++ b/shell/hush.c
@@ -1273,10 +1273,10 @@ static void get_user_input(struct in_str *i)
 	prompt_str = setup_prompt_string(i->promptmode);
 #if ENABLE_FEATURE_EDITING
 	/* Enable command line editing only while a command line
-	 * is actually being read; otherwise, we'll end up bequeathing
-	 * atexit() handlers and other unwanted stuff to our
-	 * child processes (rob@sysgo.de) */
-	r = read_line_input(prompt_str, user_input_buf, BUFSIZ-1, line_input_state);
+	 * is actually being read */
+	do {
+		r = read_line_input(prompt_str, user_input_buf, BUFSIZ-1, line_input_state);
+	} while (r == 0); /* repeat if Ctrl-C */
 	i->eof_flag = (r < 0);
 	if (i->eof_flag) { /* EOF/error detected */
 		user_input_buf[0] = EOF; /* yes, it will be truncated, it's ok */
diff --git a/shell/msh.c b/shell/msh.c
index fd287f16e..917b08a1e 100644
--- a/shell/msh.c
+++ b/shell/msh.c
@@ -42,6 +42,7 @@
 # define xmalloc(size) malloc(size)
 # define msh_main(argc,argv) main(argc,argv)
 # define safe_read(fd,buf,count) read(fd,buf,count)
+# define nonblock_safe_read(fd,buf,count) read(fd,buf,count)
 # define NOT_LONE_DASH(s) ((s)[0] != '-' || (s)[1])
 # define LONE_CHAR(s,c) ((s)[0] == (c) && !(s)[1])
 # define ATTRIBUTE_NORETURN __attribute__ ((__noreturn__))
@@ -3376,7 +3377,7 @@ static int doread(struct op *t)
 	}
 	for (wp = t->words + 1; *wp; wp++) {
 		for (cp = global_env.linep; !nl && cp < elinep - 1; cp++) {
-			nb = read(0, cp, sizeof(*cp));
+			nb = nonblock_safe_read(0, cp, sizeof(*cp));
 			if (nb != sizeof(*cp))
 				break;
 			nl = (*cp == '\n');
@@ -4799,7 +4800,7 @@ static int filechar(struct ioarg *ap)
 			if (i)
 				lseek(ap->afile, ap->afpos, SEEK_SET);
 
-			i = safe_read(ap->afile, bp->buf, sizeof(bp->buf));
+			i = nonblock_safe_read(ap->afile, bp->buf, sizeof(bp->buf));
 			if (i <= 0) {
 				closef(ap->afile);
 				return 0;
@@ -4830,7 +4831,7 @@ static int filechar(struct ioarg *ap)
 		return c;
 	}
 #endif
-	i = safe_read(ap->afile, &c, sizeof(c));
+	i = nonblock_safe_read(ap->afile, &c, sizeof(c));
 	return i == sizeof(c) ? (c & 0x7f) : (closef(ap->afile), 0);
 }
 
@@ -4841,7 +4842,7 @@ static int herechar(struct ioarg *ap)
 {
 	char c;
 
-	if (read(ap->afile, &c, sizeof(c)) != sizeof(c)) {
+	if (nonblock_safe_read(ap->afile, &c, sizeof(c)) != sizeof(c)) {
 		close(ap->afile);
 		c = '\0';
 	}