Add API to get the date at which the unit file that produced SymbolLocation was modified

This allows us to check for out-of-date index entries.

Author: ahoppen

Sources/ISDBTestSupport/TestLocation.swift

@@ -51,6 +51,7 @@ extension SymbolLocation {
   public init(_ loc: TestLocation, moduleName: String = TestLocation.unknownModuleName, isSystem: Bool = false) {
     self.init(
       path: loc.url.path,
+      timestamp: Date(),
       moduleName: moduleName,
       isSystem: isSystem,
       line: loc.line,

Sources/IndexStoreDB/SymbolLocation.swift

@@ -12,16 +12,20 @@
 
 @_implementationOnly
 import CIndexStoreDB
+import Foundation
 
 public struct SymbolLocation: Equatable {
   public var path: String
+  /// The date at which the unit file that contains a symbol has last been modified.
+  public var timestamp: Date
   public var moduleName: String
   public var isSystem: Bool
   public var line: Int
   public var utf8Column: Int
 
-  public init(path: String, moduleName: String, isSystem: Bool = false, line: Int, utf8Column: Int) {
+  public init(path: String, timestamp: Date, moduleName: String, isSystem: Bool = false, line: Int, utf8Column: Int) {
     self.path = path
+    self.timestamp = timestamp
     self.moduleName = moduleName
     self.isSystem = isSystem
     self.line = line
@@ -47,6 +51,7 @@ extension SymbolLocation: CustomStringConvertible {
 extension SymbolLocation {
   internal init(_ loc: indexstoredb_symbol_location_t) {
     path = String(cString: indexstoredb_symbol_location_path(loc))
+    timestamp = Date(timeIntervalSince1970: indexstoredb_symbol_location_timestamp(loc))
     moduleName = String(cString: indexstoredb_symbol_location_module_name(loc))
     isSystem = indexstoredb_symbol_location_is_system(loc)
     line = Int(indexstoredb_symbol_location_line(loc))

include/CIndexStoreDB/CIndexStoreDB.h

@@ -375,6 +375,12 @@ INDEXSTOREDB_PUBLIC
 const char * _Nonnull
 indexstoredb_symbol_location_path(_Nonnull indexstoredb_symbol_location_t);
 
+/// Returns a Unix timestamp (seconds since 1/1/1970) at which the unit file that contains a symbol has last been 
+/// modified.
+INDEXSTOREDB_PUBLIC
+double
+indexstoredb_symbol_location_timestamp(_Nonnull indexstoredb_symbol_location_t loc);
+
 /// Returns the module name of the given symbol location.
 ///
 /// The string has the same lifetime as the \c indexstoredb_symbol_location_t.

lib/CIndexStoreDB/CIndexStoreDB.cpp

@@ -444,6 +444,17 @@ indexstoredb_symbol_location_path(indexstoredb_symbol_location_t loc) {
   return obj->getPath().getPathString().c_str();
 }
 
+double
+indexstoredb_symbol_location_timestamp(indexstoredb_symbol_location_t loc) {
+  auto obj = (SymbolLocation *)loc;
+  // Up until C++20 the reference date of time_since_epoch is undefined but according to 
+  // https://en.cppreference.com/w/cpp/chrono/system_clock most implementations use Unix Time.
+  // Since C++20, system_clock is defined to measure time since 1/1/1970.
+  // We rely on `time_since_epoch` always returning the nanoseconds since 1/1/1970.
+  auto nanosecondsSinceEpoch = obj->getPath().getModificationTime().time_since_epoch().count();
+  return static_cast<double>(nanosecondsSinceEpoch) / 1000 / 1000 / 1000;
+}
+
 const char *
 indexstoredb_symbol_location_module_name(indexstoredb_symbol_location_t loc) {
   auto obj = (SymbolLocation *)loc;