Thanks to John Cummings.

src/cmd/upas/common/appendfiletombox.c

@@ -0,0 +1,155 @@
+#include "common.h"
+
+enum {
+	Buffersize = 64*1024,
+};
+
+typedef struct Inbuf Inbuf;
+struct Inbuf
+{
+	char buf[Buffersize];
+	char *wp;
+	char *rp;
+	int eof;
+	int in;
+	int out;
+	int last;
+	ulong bytes;
+};
+
+static Inbuf*
+allocinbuf(int in, int out)
+{
+	Inbuf *b;
+
+	b = mallocz(sizeof(Inbuf), 1);
+	if(b == nil)
+		sysfatal("reading mailbox: %r");
+	b->rp = b->wp = b->buf;
+	b->in = in;
+	b->out = out;
+	return b;
+}
+
+static int
+fill(Inbuf *b, int addspace)
+{
+	int i, n;
+
+	if(b->eof && b->wp - b->rp == 0)
+		return 0;
+
+	n = b->rp - b->buf;
+	if(n > 0){
+		i = write(b->out, b->buf, n);
+		if(i != n)
+			return -1;
+		b->last = b->buf[n-1];
+		b->bytes += n;
+	}
+	if(addspace){
+		if(write(b->out, " ", 1) != 1)
+			return -1;
+		b->last = ' ';
+		b->bytes++;
+	}
+
+	n = b->wp - b->rp;
+	memmove(b->buf, b->rp, n);
+	b->rp = b->buf;
+	b->wp = b->rp + n;
+
+	i = read(b->in, b->buf+n, sizeof(b->buf)-n);
+	if(i < 0)
+		return -1;
+	b->wp += i;
+
+	return b->wp - b->rp;
+}
+
+/* code to escape ' '*From' ' at the beginning of a line */
+int
+appendfiletombox(int in, int out)
+{
+	int addspace;
+	int n;
+	char *p;
+	int sol;
+	Inbuf *b;
+
+	seek(out, 0, 2);
+
+	b = allocinbuf(in, out);
+	addspace = 0;
+	sol = 1;
+
+	for(;;){
+		if(b->wp - b->rp < 5){
+			n = fill(b, addspace);
+			addspace = 0;
+			if(n < 0)
+				goto error;
+			if(n == 0)
+				break;
+			if(n < 5){
+				b->rp = b->wp;
+				continue;
+			}
+		}
+
+		/* state machine looking for ' '*From' ' */
+		if(!sol){
+			p = memchr(b->rp, '\n', b->wp - b->rp);
+			if(p == nil)
+				b->rp = b->wp;
+			else{
+				b->rp = p+1;
+				sol = 1;
+			}
+			continue;
+		} else {
+			if(*b->rp == ' ' || strncmp(b->rp, "From ", 5) != 0){
+				b->rp++;
+				continue;
+			}
+			addspace = 1;
+			sol = 0;
+		}
+	}
+
+	/* mailbox entries always terminate with two newlines */
+	n = b->last == '\n' ? 1 : 2;
+	if(write(out, "\n\n", n) != n)
+		goto error;
+	n += b->bytes;
+	free(b);
+	return n;
+error:
+	free(b);
+	return -1;
+}
+
+int
+appendfiletofile(int in, int out)
+{
+	int n;
+	Inbuf *b;
+
+	seek(out, 0, 2);
+
+	b = allocinbuf(in, out);
+	for(;;){
+		n = fill(b, 0);
+		if(n < 0)
+			goto error;
+		if(n == 0)
+			break;
+		b->rp = b->wp;
+	}
+	n = b->bytes;
+	free(b);
+	return n;
+error:
+	free(b);
+	return -1;
+}