as discussed on 9fans, a version of hget that doesn't seek on output.
it also takes a bit more care to make sure that it doesn't mix
up the output from several different documents. it requires the md5pickle patch.

although i've done some testing of the output routing independently,
i don't know how to provoke webservers into giving the required responses
so it needs some more testing.

Reference: /n/sources/patch/applied/hgetnoseek
Date: Mon Apr  5 23:18:23 CES 2004

--- /sys/src/cmd/hget.c	Mon Apr  5 23:18:23 2004
+++ /sys/src/cmd/hget.c	Mon Apr  5 23:18:23 2004
@@ -25,6 +25,16 @@
 	long	end;
 };
 
+typedef struct Out Out;
+struct Out
+{
+	int fd;
+	int offset;				/* notional current offset in output */
+	int written;			/* number of bytes successfully transferred to output */
+	DigestState *curr;		/* digest state up to offset (if known) */
+	DigestState *hiwat;		/* digest state of all bytes written */
+};
+
 enum
 {
 	Http,
@@ -44,8 +54,9 @@
 int debug;
 char *ofile;
 
-int	doftp(URL*, URL*, Range*, int, long);
-int	dohttp(URL*, URL*,  Range*, int, long);
+
+int	doftp(URL*, URL*, Range*, Out*, long);
+int	dohttp(URL*, URL*,  Range*, Out*, long);
 int	crackurl(URL*, char*);
 Range*	crackrange(char*);
 int	getheader(int, char*, int);
@@ -58,6 +69,8 @@
 int	readibuf(int, char*, int);
 int	dfprint(int, char*, ...);
 void	unreadline(char*);
+int	output(Out*, void*, int);
+void	setoffset(Out*, int);
 
 int	verbose;
 char	*net;
@@ -66,7 +79,7 @@
 
 struct {
 	char	*name;
-	int	(*f)(URL*, URL*, Range*, int, long);
+	int	(*f)(URL*, URL*, Range*, Out*, long);
 } method[] = {
 	[Http]	{ "http",	dohttp },
 	[Https]	{ "https",	dohttp },
@@ -86,11 +99,12 @@
 {
 	URL u;
 	Range r;
-	int fd, errs, n;
+	int errs, n;
 	ulong mtime;
 	Dir *d;
 	char postbody[4096], *p, *e, *t, *hpx;
 	URL px; // Proxy
+	Out out;
 
 	ofile = nil;
 	p = postbody;
@@ -145,16 +159,21 @@
 	if(argc != 1)
 		usage();
 
-	fd = 1;
+	
+	out.fd = 1;
+	out.written = 0;
+	out.offset = 0;
+	out.curr = nil;
+	out.hiwat = nil;
 	if(ofile != nil){
 		d = dirstat(ofile);
 		if(d == nil){
-			fd = create(ofile, OWRITE, 0664);
-			if(fd < 0)
+			out.fd = create(ofile, OWRITE, 0664);
+			if(out.fd < 0)
 				sysfatal("creating %s: %r", ofile);
 		} else {
-			fd = open(ofile, OWRITE);
-			if(fd < 0)
+			out.fd = open(ofile, OWRITE);
+			if(out.fd < 0)
 				sysfatal("can't open %s: %r", ofile);
 			r.start = d->length;
 			mtime = d->mtime;
@@ -170,10 +189,10 @@
 		sysfatal("%r");
 
 	for(;;){
+		setoffset(&out, 0);
 		/* transfer data */
 		werrstr("");
-		seek(fd, 0, 0);
-		n = (*method[u.method].f)(&u, &px, &r, fd, mtime);
+		n = (*method[u.method].f)(&u, &px, &r, &out, mtime);
 
 		switch(n){
 		case Eof:
@@ -294,7 +313,7 @@
 }
 
 int
-dohttp(URL *u, URL *px, Range *r, int out, long mtime)
+dohttp(URL *u, URL *px, Range *r, Out *out, long mtime)
 {
 	int fd, cfd;
 	int redirect, loop;
@@ -414,7 +433,7 @@
 			sysfatal("No Content");
 
 		case 206:	/* Partial Content */
-			seek(out, r->start, 0);
+			setoffset(out, r->start);
 			break;
 
 		case 301:	/* Moved Permanently */
@@ -478,7 +497,7 @@
 
 	/* transfer whatever you get */
 	if(ofile != nil && u->mtime != 0){
-		note.fd = out;
+		note.fd = out->fd;
 		note.mtime = u->mtime;
 		notify(catch);
 	}
@@ -489,7 +508,7 @@
 		n = readibuf(fd, buf, sizeof(buf));
 		if(n <= 0)
 			break;
-		if(write(out, buf, n) != n)
+		if(output(out, buf, n) != n)
 			break;
 		tot += n;
 		if(verbose && vtime != time(0)) {
@@ -506,7 +525,7 @@
 		rerrstr(err, sizeof err);
 		nulldir(&d);
 		d.mtime = u->mtime;
-		if(dirfwstat(out, &d) < 0)
+		if(dirfwstat(out->fd, &d) < 0)
 			fprint(2, "couldn't set mtime: %r\n");
 		errstr(err, sizeof err);
 	}
@@ -765,13 +784,13 @@
 int xfertype(int, char*);
 int passive(int, URL*);
 int active(int, URL*);
-int ftpxfer(int, int, Range*);
+int ftpxfer(int, Out*, Range*);
 int terminateftp(int, int);
 int getaddrport(char*, uchar*, uchar*);
-int ftprestart(int, int, URL*, Range*, long);
+int ftprestart(int, Out*, URL*, Range*, long);
 
 int
-doftp(URL *u, URL *px, Range *r, int out, long mtime)
+doftp(URL *u, URL *px, Range *r, Out *out, long mtime)
 {
 	int pid, ctl, data, rv;
 	Waitmsg *w;
@@ -937,7 +956,7 @@
 }
 
 int
-ftprestart(int ctl, int out, URL *u, Range *r, long mtime)
+ftprestart(int ctl, Out *out, URL *u, Range *r, long mtime)
 {
 	Tm tm;
 	char msg[1024];
@@ -978,9 +997,9 @@
 	/* seek to restart point */
 	if(r->start > 0){
 		ftpcmd(ctl, "REST %lud", r->start);
-		if(ftprcode(ctl, msg, sizeof(msg)) == Incomplete)
-			seek(out, r->start, 0);
-		else
+		if(ftprcode(ctl, msg, sizeof(msg)) == Incomplete){
+			setoffset(out, r->start);
+		}else
 			r->start = 0;
 	}
 
@@ -1134,7 +1153,7 @@
 }
 
 int
-ftpxfer(int in, int out, Range *r)
+ftpxfer(int in, Out *out, Range *r)
 {
 	char buf[1024];
 	long vtime;
@@ -1147,7 +1166,7 @@
 			break;
 		if(i < 0)
 			return Error;
-		if(write(out, buf, i) != i)
+		if(output(out, buf, i) != i)
 			return Error;
 		r->start += i;
 		if(verbose && vtime != time(0)) {
@@ -1321,4 +1340,79 @@
 	port[0] = i>>8;
 	port[1] = i;
 	return 0;
+}
+
+void
+md5free(DigestState *state)
+{
+	uchar x[MD5dlen];
+	md5(nil, 0, x, state);
+}
+
+DigestState*
+md5dup(DigestState *state)
+{
+	char *p;
+	p = md5pickle(state);
+	state = md5unpickle(p);
+	free(p);
+	return state;
+}
+
+void
+setoffset(Out *out, int offset)
+{
+	md5free(out->curr);
+	if(offset == 0)
+		out->curr = md5(nil, 0, nil, nil);
+	else
+		out->curr = nil;
+	out->offset = offset;
+}
+
+/*
+ * write some output, discarding it (but keeping track)
+ * if we've already written it. if we've gone backwards,
+ * verify that everything previously written matches
+ * that which would have been written from the current
+ * output.
+ */
+int
+output(Out *out, char *buf, int nb)
+{
+	int n, d;
+	uchar m0[MD5dlen], m1[MD5dlen];
+
+	n = nb;
+	d = out->written - out->offset;
+	assert(d >= 0);
+	if(d > 0){
+		if(n < d){
+			if(out->curr != nil)
+				md5((uchar*)buf, n, nil, out->curr);
+			out->offset += n;
+			return n;
+		}
+		if(out->curr != nil){
+			md5((uchar*)buf, d, m0, out->curr);
+			out->curr = nil;
+			md5(nil, 0, m1, md5dup(out->hiwat));
+			if(memcmp(m0, m1, MD5dlen) != 0){
+				fprint(2, "integrity check failure at offset %d\n", out->written);
+				return -1;
+			}
+		}
+		buf += d;
+		n -= d;
+		out->offset += d;
+	}
+	if(n > 0){
+		out->hiwat = md5((uchar*)buf, n, nil, out->hiwat);
+		n = write(out->fd, buf, n);
+		if(n > 0){
+			out->offset += n;
+			out->written += n;
+		}
+	}
+	return n + d;
 }