buffer: make clone fork()

Makes e.g. on-page anchor navigation near-instantaneous. Well, as
instantaneous as a fork can be. In any case, it's a lot faster
than loading the entire page anew.

This involves duplicating open resources (file descriptors, etc.),
which is not exactly trivial. For now we have a huge clone() procedure
that does an ok-ish job at it, but there remains a lot of room for
improvement.

e.g. cloning is still broken in some cases:

* As noted in the comments, TeeStream'ing the input stream for any
  buffer is a horrible idea, as readout in the cloned buffer now
  depends on the original buffer also reading from the stream. (So
  e.g. if you clone, then kill the old buffer without waiting for
  the new one to load, the new buffer gets stuck.)
* Timeouts/intervals are broken in cloned buffers. The timeout
  module probably needs a redesign to fix this.
* If you clone before connect2, the cloned buffer gets stuck.

The previous solution was even worse (i.e. broken in more cases),
so this is still an improvement. For example, this fixes some issues
with mailcap handling (removes the "set the Content-Type of htmloutput
buffers to text/html" hack), does not reload all resources, does not
completely break if the buffer is cloned during loading, etc.

2 files changed, 76 insertions, 20 deletions

diff --git a/src/local/container.nim b/src/local/container.nim
index f8b6f1fc..d191c6b3 100644
--- a/src/local/container.nim
+++ b/src/local/container.nim
@@ -98,6 +98,7 @@ type
     redirectdepth*: int
     select*: Select
     canreinterpret*: bool
+    cloned: bool
 
 jsDestructor(Container)
 
@@ -129,15 +130,62 @@ proc newBuffer*(forkserver: ForkServer, mainproc: Pid, config: BufferConfig,
     canreinterpret: canreinterpret
   )
 
+func location*(container: Container): URL {.jsfget.} =
+  return container.source.location
+
+proc clone*(container: Container, newurl: URL): Promise[Container] =
+  let url = if newurl != nil:
+    newurl
+  else:
+    container.location
+  return container.iface.clone(url).then(proc(pid: Pid): Container =
+    if pid == -1:
+      return nil
+    let ncontainer = Container(
+      config: container.config,
+      iface: container.iface, # changed later in setStream
+      width: container.width,
+      height: container.height,
+      title: container.title,
+      hovertext: container.hovertext,
+      lastpeek: container.lastpeek,
+      source: container.source,
+      pos: container.pos,
+      bpos: container.bpos,
+      process: pid,
+      loadinfo: container.loadinfo,
+      lines: container.lines,
+      lineshift: container.lineshift,
+      numLines: container.numLines,
+      code: container.code,
+      retry: container.retry,
+      hlon: container.hlon,
+      redraw: container.redraw,
+      #needslines: container.needslines,
+      canceled: container.canceled,
+      events: container.events,
+      startpos: container.startpos,
+      hasstart: container.hasstart,
+      redirectdepth: container.redirectdepth,
+      select: container.select,
+      canreinterpret: container.canreinterpret,
+      cloned: true
+    )
+    for hl in container.highlights:
+      var hl0 = Highlight()
+      hl0[] = hl[]
+      ncontainer.highlights.add(hl0)
+    if newurl != nil:
+      ncontainer.source.location = newurl
+    return ncontainer
+  )
+
 func charset*(container: Container): Charset =
   return container.source.charset
 
 func contentType*(container: Container): Option[string] {.jsfget.} =
   return container.source.contenttype
 
-func location*(container: Container): URL {.jsfget.} =
-  return container.source.location
-
 func lineLoaded(container: Container, y: int): bool =
   return y - container.lineshift in 0..container.lines.high
 
@@ -931,12 +979,18 @@ proc handleCommand(container: Container) =
   container.iface.resolve(packetid, len - slen(packetid))
 
 proc setStream*(container: Container, stream: Stream) =
-  container.iface = newBufferInterface(stream)
-  if container.source.t == LOAD_PIPE:
-    container.iface.passFd(container.source.fd).then(proc() =
-      discard close(container.source.fd))
-    stream.flush()
-  container.load()
+  if not container.cloned:
+    container.iface = newBufferInterface(stream)
+    if container.source.t == LOAD_PIPE:
+      container.iface.passFd(container.source.fd).then(proc() =
+        discard close(container.source.fd))
+      stream.flush()
+    container.load()
+  else:
+    container.iface = container.iface.clone(stream)
+    # Maybe we have to resume loading. Let's try.
+    discard container.iface.load().then(proc(res: LoadResult) =
+      container.onload(res))
 
 proc onreadline(container: Container, w: Slice[int], handle: (proc(line: SimpleFlexibleLine)), res: GetLinesResult) =
   for line in res.lines:
diff --git a/src/local/pager.nim b/src/local/pager.nim
index 26cbba53..d6845636 100644
--- a/src/local/pager.nim
+++ b/src/local/pager.nim
@@ -406,7 +406,7 @@ proc newBuffer(pager: Pager, bufferConfig: BufferConfig, source: BufferSource,
     canreinterpret
   )
 
-proc dupeBuffer(pager: Pager, container: Container, location: URL,
+proc dupeBuffer2(pager: Pager, container: Container, location: URL,
     contentType = ""): Container =
   let contentType = if contentType != "":
     some(contentType)
@@ -426,8 +426,16 @@ proc dupeBuffer(pager: Pager, container: Container, location: URL,
   container.pipeBuffer(pipeTo)
   return pipeTo
 
-proc dupeBuffer(pager: Pager, location: URL = nil) {.jsfunc.} =
-  pager.addContainer(pager.dupeBuffer(pager.container, location))
+proc dupeBuffer(pager: Pager, container: Container, location: URL) =
+  container.clone(location).then(proc(container: Container) =
+    if container == nil:
+      pager.alert("Failed to duplicate buffer.")
+    else:
+      pager.addContainer(container)
+  )
+
+proc dupeBuffer(pager: Pager) {.jsfunc.} =
+  pager.dupeBuffer(pager.container, pager.container.location)
 
 # The prevBuffer and nextBuffer procedures emulate w3m's PREV and NEXT
 # commands by traversing the container tree in a depth-first order.
@@ -560,7 +568,7 @@ proc toggleSource(pager: Pager) {.jsfunc.} =
       "text/plain"
     else:
       "text/html"
-    let container = pager.dupeBuffer(pager.container, nil, contenttype)
+    let container = pager.dupeBuffer2(pager.container, nil, contenttype)
     container.sourcepair = pager.container
     pager.container.sourcepair = container
     pager.addContainer(container)
@@ -897,9 +905,6 @@ proc runMailcapReadPipe(pager: Pager, container: Container,
   let p2 = p.then(proc(): auto =
     discard close(fdin)
     let ishtml = HTMLOUTPUT in entry.flags
-    if ishtml:
-      #TODO this is a hack for dupe buffer and should be reconsidered.
-      container.source.contenttype = some("text/html")
     return container.readFromFd(fdout, ishtml)
   ).then(proc() =
     discard close(fdout)
@@ -972,9 +977,6 @@ proc runMailcapReadFile(pager: Pager, container: Container,
     discard close(pipefd[1])
     let fdout = pipefd[0]
     let ishtml = HTMLOUTPUT in entry.flags
-    if ishtml:
-      #TODO this is a hack for dupe buffer and should be reconsidered.
-      container.source.contenttype = some("text/html")
     return container.readFromFd(fdout, ishtml).then(proc() =
       discard close(fdout)
     )
@@ -1118,7 +1120,7 @@ proc handleEvent0(pager: Pager, container: Container, event: ContainerEvent): bo
   of ANCHOR:
     var url2 = newURL(container.source.location)
     url2.setHash(event.anchor)
-    pager.addContainer(pager.dupeBuffer(container, url2))
+    pager.dupeBuffer(container, url2)
   of NO_ANCHOR:
     pager.alert("Couldn't find anchor " & event.anchor)
   of UPDATE: