Use read_fd to get pipe command from stdin instead of ad-hoc code

read_fd will also now throw on read error instead of just returning
the data read so far as if nothing failed.

src/file.cc

@@ -41,6 +41,9 @@ public:
     file_access_error(StringView filename,
                       StringView error_desc)
         : runtime_error(format("{}: {}", filename, error_desc)) {}
+
+    file_access_error(int fd, StringView error_desc)
+        : runtime_error(format("fd {}: {}", fd, error_desc)) {}
 };
 
 String parse_filename(StringView filename)
@@ -168,11 +171,10 @@ String read_fd(int fd, bool text)
     String content;
     constexpr size_t bufsize = 256;
     char buf[bufsize];
-    while (true)
+    while (ssize_t size = read(fd, buf, bufsize))
     {
-        ssize_t size = read(fd, buf, bufsize);
+        if (size == -1)
-        if (size == -1 or size == 0)
+            throw file_access_error{fd, strerror(errno)};
-            break;
 
         if  (text)
         {

src/main.cc

@@ -744,20 +744,9 @@ int run_filter(StringView keystr, StringView commands, ConstArrayView<StringView
 
 int run_pipe(StringView session)
 {
-    char buf[512];
-    String command;
-    while (ssize_t count = read(0, buf, 512))
-    {
-        if (count < 0)
-        {
-            write_stderr("error while reading stdin\n");
-            return -1;
-        }
-        command += StringView{buf, buf + count};
-    }
     try
     {
-        send_command(session, command);
+        send_command(session, read_fd(0));
     }
     catch (disconnected& e)
     {