some error fixes in signify

[Marc Espie]

Revisiting the error messages:
- pass the filenames to the low-level functions, so they can tell you
what's going on.

- FIX readall and writeall. The logic *is* wrong. Just because they
return something != len, doesn't mean they return -1.

Okay ?

Index: signify.c
===================================================================
RCS file: /build/data/openbsd/cvs/src/usr.bin/signify/signify.c,v
retrieving revision 1.6
diff -u -p -r1.6 signify.c
--- signify.c   1 Jan 2014 17:50:33 -0000       1.6
+++ signify.c   2 Jan 2014 13:37:38 -0000
@@ -92,10 +92,14 @@ xmalloc(size_t len)
 }
 
 static void
-readall(int fd, void *buf, size_t len)
+readall(int fd, void *buf, size_t len, const char *filename)
 {
-       if (read(fd, buf, len) != len)
-               err(1, "read");
+       ssize_t x = read(fd, buf, len);
+       if (x == -1) {
+               err(1, "read from %s", filename);
+       } else if (x != len) {
+               errx(1, "short read from %s", filename);
+       }
 }
 
 static void
@@ -108,7 +112,7 @@ readb64file(const char *filename, void *
        memset(b64, 0, sizeof(b64));
        rv = read(fd, b64, sizeof(b64) - 1);
        if (rv == -1)
-               err(1, "read in %s", filename);
+               err(1, "read from %s", filename);
        for (i = 0; i < rv; i++)
                if (b64[i] == '\n')
                        break;
@@ -137,7 +141,7 @@ readmsg(const char *filename, unsigned l
        if (msglen > (1UL << 30))
                errx(1, "msg too large in %s", filename);
        msg = xmalloc(msglen);
-       readall(fd, msg, msglen);
+       readall(fd, msg, msglen, filename);
        close(fd);
 
        *msglenp = msglen;
@@ -145,10 +149,14 @@ readmsg(const char *filename, unsigned l
 }
 
 static void
-writeall(int fd, const void *buf, size_t len)
+writeall(int fd, const void *buf, size_t len, const char *filename)
 {
-       if (write(fd, buf, len) != len)
-               err(1, "write");
+       ssize_t x = write(fd, buf, len);
+       if (x == -1) {
+               err(1, "write to %s", filename);
+       } else if (x != len) {
+               errx(1, "short write to %s", filename);
+       }
 }
 
 static void
@@ -161,10 +169,10 @@ writeb64file(const char *filename, const
 
        fd = xopen(filename, O_CREAT|O_EXCL|O_NOFOLLOW|O_RDWR, mode);
        snprintf(header, sizeof(header), "signify -- %s\n", comment);
-       writeall(fd, header, strlen(header));
+       writeall(fd, header, strlen(header), filename);
        if ((rv = b64_ntop(buf, len, b64, sizeof(b64))) == -1)
                errx(1, "b64 encode failed");
-       writeall(fd, b64, rv);
+       writeall(fd, b64, rv, filename);
        memset(b64, 0, sizeof(b64));
        close(fd);
 }