From 2f647017b929e5523bab586f1a5b0965503d5d21 Mon Sep 17 00:00:00 2001
From: Brian Picciano <mediocregopher@gmail.com>
Date: Fri, 30 Nov 2018 18:50:23 -0500
Subject: [PATCH] mlog: add CtxSet and From functions

---
 mlog/ctx.go      | 77 ++++++++++++++++++++++++++++++++++++++++++++++++
 mlog/ctx_test.go | 39 ++++++++++++++++++++++++
 2 files changed, 116 insertions(+)
 create mode 100644 mlog/ctx.go
 create mode 100644 mlog/ctx_test.go

diff --git a/mlog/ctx.go b/mlog/ctx.go
new file mode 100644
index 0000000..f4880b8
--- /dev/null
+++ b/mlog/ctx.go
@@ -0,0 +1,77 @@
+package mlog
+
+import (
+	"path"
+
+	"github.com/mediocregopher/mediocre-go-lib/mctx"
+)
+
+type ctxKey int
+
+// CtxSet caches the Logger in the Context, overwriting any previous one which
+// might have been cached there. From is the corresponding function which
+// retrieves the Logger back out when needed.
+//
+// This function can be used to premptively set a preconfigured Logger on a root
+// Context so that the default (NewLogger) isn't used when From is called for
+// the first time.
+func CtxSet(ctx mctx.Context, l *Logger) {
+	mctx.GetSetMutableValue(ctx, false, ctxKey(0), func(interface{}) interface{} {
+		return l
+	})
+}
+
+type ctxPathStringer struct {
+	str     Stringer
+	pathStr string
+}
+
+func (cp ctxPathStringer) String() string {
+	return "(" + cp.pathStr + ") " + cp.str.String()
+}
+
+// From returns an instance of Logger which has been customized for this
+// Context, primarily by adding a prefix describing the Context's path to all
+// Message descriptions the Logger receives.
+//
+// The Context caches within it the generated Logger, so a new one isn't created
+// everytime. When From is first called on a Context the Logger inherits the
+// Context parent's Logger. If the parent hasn't had From called on it its
+// parent will be queried instead, and so on.
+func From(ctx mctx.Context) *Logger {
+	var from func(mctx.Context) *Logger
+	from = func(ctx mctx.Context) *Logger {
+		return mctx.GetSetMutableValue(ctx, true, ctxKey(0), func(interface{}) interface{} {
+
+			ctxPath := mctx.Path(ctx)
+			if len(ctxPath) == 0 {
+				// we're at the root node and it doesn't have a Logger set, use
+				// the default
+				return NewLogger()
+			}
+
+			// set up child's logger
+			pathStr := "/"
+			if len(ctxPath) > 0 {
+				pathStr += path.Join(ctxPath...)
+			}
+
+			parentL := from(mctx.Parent(ctx))
+			prevH := parentL.Handler()
+			return parentL.WithHandler(func(msg Message) error {
+				// if the Description is already a ctxPathStringer it can be
+				// assumed this Message was passed in from a child Logger.
+				if _, ok := msg.Description.(ctxPathStringer); !ok {
+					msg.Description = ctxPathStringer{
+						str:     msg.Description,
+						pathStr: pathStr,
+					}
+				}
+				return prevH(msg)
+			})
+
+		}).(*Logger)
+	}
+
+	return from(ctx)
+}
diff --git a/mlog/ctx_test.go b/mlog/ctx_test.go
new file mode 100644
index 0000000..90eb0f2
--- /dev/null
+++ b/mlog/ctx_test.go
@@ -0,0 +1,39 @@
+package mlog
+
+import (
+	. "testing"
+
+	"github.com/mediocregopher/mediocre-go-lib/mctx"
+	"github.com/mediocregopher/mediocre-go-lib/mtest/massert"
+)
+
+func TestContextStuff(t *T) {
+	ctx := mctx.New()
+	ctx1 := mctx.ChildOf(ctx, "1")
+	ctx1a := mctx.ChildOf(ctx1, "a")
+	ctx1b := mctx.ChildOf(ctx1, "b")
+
+	var descrs []string
+	l := NewLogger().WithHandler(func(msg Message) error {
+		descrs = append(descrs, msg.Description.String())
+		return nil
+	})
+	CtxSet(ctx, l)
+
+	From(ctx1a).Info("ctx1a")
+	From(ctx1).Info("ctx1")
+	From(ctx).Info("ctx")
+	From(ctx1b).Info("ctx1b")
+
+	ctx2 := mctx.ChildOf(ctx, "2")
+	From(ctx2).Info("ctx2")
+
+	massert.Fatal(t, massert.All(
+		massert.Len(descrs, 5),
+		massert.Equal(descrs[0], "(/1/a) ctx1a"),
+		massert.Equal(descrs[1], "(/1) ctx1"),
+		massert.Equal(descrs[2], "ctx"),
+		massert.Equal(descrs[3], "(/1/b) ctx1b"),
+		massert.Equal(descrs[4], "(/2) ctx2"),
+	))
+}