From 3d5b3b8016e1bc932c386c46a4949e178d5985b9 Mon Sep 17 00:00:00 2001
From: Wolfgang Draxinger <code@datenwolf.net>
Date: Thu, 20 Oct 2022 12:28:09 +0200
Subject: Initial commit

---
 Makefile   | 10 ++++++++++
 README     | 18 ++++++++++++++++++
 src/rsvr.c | 60 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 88 insertions(+)
 create mode 100644 Makefile
 create mode 100644 README
 create mode 100644 src/rsvr.c

diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..52b516d
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,10 @@
+.PHONY: all clean
+
+all: install/bin/rsvr
+
+clean:
+	-rm -r install
+
+install/bin/rsvr: src/rsvr.c
+	mkdir -p install/bin
+	$(CC) -Os -Wl,-s -o $@ $<
diff --git a/README b/README
new file mode 100644
index 0000000..356ab49
--- /dev/null
+++ b/README
@@ -0,0 +1,18 @@
+# rsvr -- a in-memory file reservoir
+
+rsvr is a small helper utility that receives data from non-seekable
+sources on `stdin` such as a pipe or a socket, placing it into a in-memory
+file resorvoir (it also can read from regular, seekable files). When the
+source is closed (or end-of-file is reached) the subsequent command is
+executed.
+
+There are no options to set. Usage is as simple as
+
+    rsvr <subsequent command with argument>
+
+With the source data being read from `stdin`.
+
+# Example
+
+    # download a PDF from the web and show it using MuPDF
+    curl <some URL> | rsvr mupdf /dev/stdin
diff --git a/src/rsvr.c b/src/rsvr.c
new file mode 100644
index 0000000..01fa845
--- /dev/null
+++ b/src/rsvr.c
@@ -0,0 +1,60 @@
+#define _GNU_SOURCE
+#include <sys/mman.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <string.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <stdio.h>
+
+char buf[128*1024];
+
+int main(int argc, char *argv[])
+{
+	if( 2 > argc ){
+		fprintf(stderr,
+			"Read stdin into a memfd reservoir file and use this as stdin for a subsequently executed command.\n"
+			"usage: %s <command and argument>\n",
+			(argc ? argv[0] : "rsvr") );
+		return EXIT_FAILURE;
+	}
+
+	int rsvr_fd = memfd_create("reservoiur", MFD_CLOEXEC);
+	if( 0 > rsvr_fd ){
+		perror("memfd_create('reservoiur', MFD_CLOEXEC)"); 
+		return EXIT_FAILURE;
+	}
+
+	ssize_t rb = 0, wb = 0;
+	do {
+		rb = read(0, buf, sizeof(buf));
+		if( 0 >= rb ){ continue; }
+
+		for( ssize_t cb = 0
+		   ; 0 <= wb && (cb < rb)
+		   ; cb += ((0 < wb) ? wb : 0)
+		){
+			wb = write(rsvr_fd, (buf+cb), (rb-cb));
+			if( (0 > wb) && (EINTR == errno) ){ wb = 0; }
+		}
+	} while( (0 < rb)
+	      || (0 > rb) && EINTR == errno );
+
+	if( 0 > rb ){ perror("read(stdin)"); }
+	if( 0 > wb ){ perror("write(reservoir)"); }
+	if( 0 > rb || 0 > wb ){ return EXIT_FAILURE; }
+
+	if( 0 > dup2(rsvr_fd, 0) ){
+		perror("dup2(reservoir, stdin)");
+		return EXIT_FAILURE;
+	}
+	close(rsvr_fd);
+
+	memmove(argv, argv+1, sizeof(*argv)*(argc-1));
+	argv[argc-1] = NULL;
+	if( 0 > execvp(argv[0], argv) ){
+		perror("exec(...)");
+	}
+
+	return EXIT_FAILURE;
+}
-- 
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