Add new helper class pipe reader

Implements synchronous, non-block reading from pipes.

include/linuxdeploy/subprocess/pipe_reader.h

@@ -0,0 +1,36 @@
+#pragma once
+
+#include <string>
+#include <set>
+#include <vector>
+#include <poll.h>
+
+/**
+ * Reads from a pipe when data is available, and hands data to registered callbacks.
+ */
+class pipe_reader {
+private:
+    const int pipe_fd_;
+
+public:
+    /**
+     * Construct new instance from pipe file descriptor.
+     * @param pipe_fd file descriptor for pipe we will read from (e.g., a subprocess's stdout, stderr pipes)
+     */
+    explicit pipe_reader(int pipe_fd);
+
+    /**
+     * Read from pipe file descriptor and store data in buffer.
+     * This method is non-blocking, i.e., if there is no data to read, it returns immediately.
+     * Otherwise, it will read until either of the following conditions is met:
+     *
+     *     - no more data left in the pipe to be read
+     *     - buffer is completely filled
+     *
+     * On errors, a subprocess_error is thrown.
+     *
+     * @param buffer buffer to store read data into
+     * @returns amount of characters read from the pipe
+     */
+    size_t read(std::vector<std::string::value_type>& buffer) const;
+};

src/subprocess/pipe_reader.cpp

@@ -0,0 +1,31 @@
+// system headers
+#include <algorithm>
+#include <fcntl.h>
+#include <unistd.h>
+#include <functional>
+#include <cstring>
+
+// local headers
+#include "linuxdeploy/subprocess/pipe_reader.h"
+
+pipe_reader::pipe_reader(int pipe_fd) : pipe_fd_(pipe_fd) {
+    // add O_NONBLOCK TO fd's flags to be able to read
+    auto flags = fcntl(pipe_fd_, F_GETFL, 0);
+    flags |= O_NONBLOCK;
+    fcntl(pipe_fd_, F_SETFL, flags);
+}
+
+size_t pipe_reader::read(std::vector<std::string::value_type>& buffer) const {
+    ssize_t rv = ::read(pipe_fd_, buffer.data(), buffer.size());
+
+    if (rv == -1) {
+        // no data available
+        if (errno == EAGAIN)
+            return 0;
+
+        // TODO: introduce custom subprocess_error
+        throw std::runtime_error{"unexpected error reading from pipe: " + std::string(strerror(errno))};
+    }
+
+    return rv;
+}