Add error tracing utility

libimagutil/src/lib.rs

@@ -2,4 +2,5 @@
 extern crate regex;
 
 pub mod key_value_split;
+pub mod trace;
 pub mod variants;

libimagutil/src/trace.rs

@@ -0,0 +1,46 @@
+use std::error::Error;
+use std::io::Write;
+use std::io::stderr;
+
+pub fn trace_error(e: &Error) {
+    print_trace_maxdepth(count_error_causes(e), e, ::std::u64::MAX);
+    write!(stderr(), "");
+}
+
+pub fn trace_error_maxdepth(e: &Error, max: u64) {
+    let n = count_error_causes(e);
+    write!(stderr(), "{}/{} Levels of errors will be printed", (if max > n { n } else { max }), n);
+    print_trace_maxdepth(n, e, max);
+    write!(stderr(), "");
+}
+
+pub fn trace_error_dbg(e: &Error) {
+    print_trace_dbg(0, e);
+}
+
+/// Helper function for `trace_error()` and `trace_error_maxdepth()`.
+///
+/// Returns the cause of the last processed error in the recursion, so `None` if all errors where
+/// processed.
+fn print_trace_maxdepth(idx: u64, e: &Error, max: u64) -> Option<&Error> {
+    if e.cause().is_some() && idx > 0 {
+        print_trace_maxdepth(idx - 1, e.cause().unwrap(), max);
+        write!(stderr(), " -- caused:");
+    }
+    write!(stderr(), "Error {:>4} : {}", idx, e.description());
+    e.cause()
+}
+
+/// Count errors in Error::cause() recursively
+fn count_error_causes(e: &Error) -> u64 {
+    1 + if e.cause().is_some() { count_error_causes(e.cause().unwrap()) } else { 0 }
+}
+
+fn print_trace_dbg(idx: u64, e: &Error) {
+    debug!("Error {:>4} : {}", idx, e.description());
+    if e.cause().is_some() {
+        debug!(" -- caused by:");
+        print_trace_dbg(idx + 1, e.cause().unwrap());
+    }
+}
+