Merge branch 'ds/advice-sparse-index-expansion'

A new warning message is issued when a command has to expand a
sparse index to handle working tree cruft that are outside of the
sparse checkout.

* ds/advice-sparse-index-expansion:
  advice: warn when sparse index expands

sparse-index.c

@@ -12,6 +12,22 @@
 #include "config.h"
 #include "dir.h"
 #include "fsmonitor-ll.h"
+#include "advice.h"
+
+/**
+ * This global is used by expand_index() to determine if we should give the
+ * advice for advice.sparseIndexExpanded when expanding a sparse index to a full
+ * one. However, this is sometimes done on purpose, such as in the sparse-checkout
+ * builtin, even when index.sparse=false. This may be disabled in
+ * convert_to_sparse().
+ */
+static int give_advice_on_expansion = 1;
+#define ADVICE_MSG \
+	"The sparse index is expanding to a full index, a slow operation.\n"   \
+	"Your working directory likely has contents that are outside of\n"     \
+	"your sparse-checkout patterns. Use 'git sparse-checkout list' to\n"   \
+	"see your sparse-checkout definition and compare it to your working\n" \
+	"directory contents. Running 'git clean' may assist in this cleanup."
 
 struct modify_index_context {
 	struct index_state *write;
@@ -183,6 +199,12 @@ int convert_to_sparse(struct index_state *istate, int flags)
 	    !is_sparse_index_allowed(istate, flags))
 		return 0;
 
+	/*
+	 * If we are purposefully collapsing a full index, then don't give
+	 * advice when it is expanded later.
+	 */
+	give_advice_on_expansion = 0;
+
 	/*
 	 * NEEDSWORK: If we have unmerged entries, then stay full.
 	 * Unmerged entries prevent the cache-tree extension from working.
@@ -328,6 +350,12 @@ void expand_index(struct index_state *istate, struct pattern_list *pl)
 			pl = NULL;
 	}
 
+	if (!pl && give_advice_on_expansion) {
+		give_advice_on_expansion = 0;
+		advise_if_enabled(ADVICE_SPARSE_INDEX_EXPANDED,
+				  _(ADVICE_MSG));
+	}
+
 	/*
 	 * A NULL pattern set indicates we are expanding a full index, so
 	 * we use a special region name that indicates the full expansion.