libutil: Add bindPath function from libstore

bindPath/doBind is a useful function in build that is used in several
parts of LocalDerivationGoal. Moving this function makes it easier to
split LocalDerivationGoal implementation between several files.

Change-Id: Ic5a0768479c153c1aa3ed425f12604b20bbf0f42

3 files changed, 61 insertions, 0 deletions

diff --git a/src/libutil/meson.build b/src/libutil/meson.build
index c860e7e00..731521bbb 100644
--- a/src/libutil/meson.build
+++ b/src/libutil/meson.build
@@ -21,6 +21,7 @@ libutil_sources = files(
   'hilite.cc',
   'json-utils.cc',
   'logging.cc',
+  'mount.cc',
   'namespaces.cc',
   'position.cc',
   'print-elided.cc',
@@ -84,6 +85,7 @@ libutil_headers = files(
   'logging-json.hh',
   'lru-cache.hh',
   'monitor-fd.hh',
+  'mount.hh',
   'namespaces.hh',
   'pool.hh',
   'position.hh',
diff --git a/src/libutil/mount.cc b/src/libutil/mount.cc
new file mode 100644
index 000000000..025ced5b9
--- /dev/null
+++ b/src/libutil/mount.cc
@@ -0,0 +1,43 @@
+#include "mount.hh"
+#include "error.hh"
+#include "file-system.hh"
+#include "logging.hh"
+#if __linux__
+#include <sys/mount.h>
+
+namespace nix {
+
+void bindPath(const Path & source, const Path & target, bool optional) {
+    debug("bind mounting '%1%' to '%2%'", source, target);
+
+    auto bindMount = [&]() {
+        if (mount(source.c_str(), target.c_str(), "", MS_BIND | MS_REC, 0) == -1)
+            throw SysError("bind mount from '%1%' to '%2%' failed", source, target);
+    };
+
+    auto maybeSt = maybeLstat(source);
+    if (!maybeSt) {
+        if (optional)
+            return;
+        else
+            throw SysError("getting attributes of path '%1%'", source);
+    }
+    auto st = *maybeSt;
+
+    if (S_ISDIR(st.st_mode)) {
+        createDirs(target);
+        bindMount();
+    } else if (S_ISLNK(st.st_mode)) {
+        // Symlinks can (apparently) not be bind-mounted, so just copy it
+        createDirs(dirOf(target));
+        copyFile(source, target, {});
+    } else {
+        createDirs(dirOf(target));
+        writeFile(target, "");
+        bindMount();
+    }
+}
+
+}
+
+#endif
diff --git a/src/libutil/mount.hh b/src/libutil/mount.hh
new file mode 100644
index 000000000..59e2218be
--- /dev/null
+++ b/src/libutil/mount.hh
@@ -0,0 +1,16 @@
+#pragma once
+///@file
+
+#include "types.hh"
+
+#if __linux__
+namespace nix {
+
+/**
+ * Bind-mount file or directory from `source` to `destination`.
+ * If source does not exist this will fail unless `optional` is set
+ */
+void bindPath(const Path & source, const Path & target, bool optional = false);
+
+}
+#endif