From 67c4ed071971b1428a3004806e58d334945f197d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Deuc=D0=B5?= <shurd@sasktel.net>
Date: Thu, 4 Mar 2021 20:03:45 -0500
Subject: [PATCH] Implement POSIX requirements for FD/Stream interactions.
Documented here: https://pubs.opengroup.org/onlinepubs/009695399/functions/xsh_chap02_05.html#tag_02_05_01
This provides the best chance at read_raw() reading from the right
location and updating the file pointer as expected while avoiding
behaviour which is explicitly undefined by POSIX.
---
src/sbbs3/js_file.c | 38 ++++++++++++++++++++++++++++++++++++++
1 file changed, 38 insertions(+)
diff --git a/src/sbbs3/js_file.c b/src/sbbs3/js_file.c
index bfc4a03924..9c395972dd 100644
--- a/src/sbbs3/js_file.c
+++ b/src/sbbs3/js_file.c
@@ -355,6 +355,8 @@ js_raw_read(JSContext *cx, uintN argc, jsval *arglist)
JSString* str;
private_t* p;
jsrefcount rc;
+ int fd;
+ off_t pos;
JS_SET_RVAL(cx, arglist, JSVAL_NULL);
@@ -378,7 +380,43 @@ js_raw_read(JSContext *cx, uintN argc, jsval *arglist)
return(JS_TRUE);
rc=JS_SUSPENDREQUEST(cx);
+ // https://pubs.opengroup.org/onlinepubs/009695399/functions/xsh_chap02_05.html#tag_02_05_01
+ /* For the first handle, the first applicable condition below applies. After the actions
+ * required below are taken, if the handle is still open, the application can close it.
+ * If it is a file descriptor, no action is required.
+ * If the only further action to be performed on any handle to this open file descriptor
+ * is to close it, no action need be taken.
+ * If it is a stream which is unbuffered, no action need be taken.
+ * If it is a stream which is line buffered, and the last byte written to the stream was a
+ * <newline> (that is, as if a: putc('\n') was the most recent operation on that stream),
+ * no action need be taken.
+ * If it is a stream which is open for writing or appending (but not also open for reading),
+ * the application shall either perform an fflush(), or the stream shall be closed.
+ * If the stream is open for reading and it is at the end of the file ( feof() is true), no
+ * action need be taken.
+ * If the stream is open with a mode that allows reading and the underlying open file
+ * description refers to a device that is capable of seeking, the application shall
+ * either perform an fflush(), or the stream shall be closed.
+ * Otherwise, the result is undefined.
+ * For the second handle:
+ * If any previous active handle has been used by a function that explicitly changed the file
+ * offset, except as required above for the first handle, the application shall perform an
+ * lseek() or fseek() (as appropriate to the type of handle) to an appropriate location.
+ */
+ /*
+ * Since we don't want to overcomplicate this, it basically boils down to:
+ * Call fflush() on the stream, lseek() on the descriptor, diddle the descriptor, then fseek() the
+ * stream.
+ *
+ * The only option bit is the fflush() on the stream, but it never hurts and is sometimes
+ * required by POSIX.
+ */
+ fflush(p->fp);
+ pos = ftell(p->fp);
+ fd = fileno(p->fp);
+ lseek(fd, pos, SEEK_SET);
len = read(fileno(p->fp),buf,len);
+ fseek(p->fp, pos + (len >= 0 ? len : 0), SEEK_SET);
dbprintf(FALSE, p, "read %u raw bytes",len);
if(len<0)
len=0;
--
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