Diagnose failure to open depdb

The cause is often a missing fsdir{} if the user tries to stash the
target in a subdirectory.

1 files changed, 8 insertions, 1 deletions

diff --git a/build2/depdb.hxx b/build2/depdb.hxx
index 520642e..6c68d19 100644
--- a/build2/depdb.hxx
+++ b/build2/depdb.hxx
@@ -14,7 +14,8 @@
 namespace build2
 {
   // Auxiliary dependency database (those .d files). Uses io_error and
-  // system_error exceptions to signal errors.
+  // system_error exceptions to signal errors except for openning (see
+  // below).
   //
   // This is a strange beast: a line-oriented, streaming database that can, at
   // some point, be switched from reading to (over)writing. The idea is to
@@ -64,6 +65,12 @@ namespace build2
     // has wrong format version, or is corrupt, then the database will be
     // immediately switched to writing.
     //
+    // If the database cannot be opened, issue diagnostics and throw failed.
+    // This commonly happens when the user tries to stash the target in a
+    // non-existent subdirectory but forgets to add the corresponding fsdir{}
+    // prerequisite. Handling this as io_error in every rule that uses depdb
+    // would be burdensome thus we issue the diagnostics here.
+    //
     depdb (const path&);
 
     // Return the modification time of the database. This value only makes