libutil: guess or invent a path from file descriptors

This is useful for certain error recovery paths (no pun intended) that
does not thread through the original path name.

Change-Id: I2d800740cb4f9912e64c923120d3f977c58ccb7e
Signed-off-by: Raito Bezarius <raito@lix.systems>

src/libutil/file-descriptor.cc

@@ -1,5 +1,8 @@
 #include "nix/util/serialise.hh"
 #include "nix/util/util.hh"
+#include "nix/util/file-system.hh"
+
+#include "util-config-private.hh"
 
 #include <fcntl.h>
 #include <unistd.h>
@@ -74,6 +77,29 @@ Descriptor AutoCloseFD::get() const
     return fd;
 }
 
+std::string guessOrInventPathFromFD(Descriptor fd)
+ {
+     assert(fd >= 0);
+     /* On Linux, there's no F_GETPATH available.
+      * But we can read /proc/ */
+ #if defined(__linux__)
+     try {
+         return readLink(fmt("/proc/self/fd/%1%", fd));
+     } catch (...) {
+     }
+ #elif defined (HAVE_F_GETPATH) && HAVE_F_GETPATH
+     std::string fdName(PATH_MAX, '\0');
+     if (fcntl(fd, F_GETPATH, fdName.data()) != -1) {
+         fdName.resize(strlen(fdName.c_str()));
+         return fdName;
+     }
+ #else
+ #error "No implementation for retrieving file descriptors path."
+ #endif
+
+     return fmt("<fd %i>", fd);
+ }
+
 
 void AutoCloseFD::close()
 {

src/libutil/include/nix/util/file-descriptor.hh

@@ -106,6 +106,14 @@ void drainFD(
 #endif
     );
 
+ /*
+  * Will attempt to guess *A* path associated that might lead to the same file as used by this
+  * file descriptor.
+  *
+  * The returned string should NEVER be used as a valid path.
+  */
+ std::string guessOrInventPathFromFD(Descriptor fd);
+
 /**
  * Get [Standard Input](https://en.wikipedia.org/wiki/Standard_streams#Standard_input_(stdin))
  */
@@ -160,6 +168,15 @@ public:
     AutoCloseFD& operator =(const AutoCloseFD & fd) = delete;
     AutoCloseFD& operator =(AutoCloseFD&& fd);
     Descriptor get() const;
+
+    /**
+     * Will attempt to guess *A* path associated that might lead to the same file as used by this
+     * file descriptor.
+     *
+     * The returned string should NEVER be used as a valid path.
+     */
+    std::string guessOrInventPath() const { return guessOrInventPathFromFD(fd); }
+
     explicit operator bool() const;
     Descriptor release();
     void close();

src/libutil/meson.build

@@ -37,6 +37,8 @@ foreach funcspec : check_funcs
   configdata.set(define_name, define_value, description: funcspec[1])
 endforeach
 
+configdata.set('HAVE_F_GETPATH', cxx.has_header_symbol('fcntl.h', 'F_GETPATH').to_int())
+
 subdir('nix-meson-build-support/libatomic')
 
 if host_machine.system() == 'windows'