From eabe2e88553e055af80c24f476bd5922e70d0776 Mon Sep 17 00:00:00 2001
From: ridiculousfish <corydoras@ridiculousfish.com>
Date: Tue, 28 Apr 2020 09:55:58 -0700
Subject: [PATCH] Allow eval to see the tty if its output is not piped

Commit 5fccfd83ece8a3126f6dc65be7b85aefc7424f14, with the fix for #6806,
switched eval to buffer its output (like other builtins do). But this
prevents using eval with commands that wants to see the tty, especially
fzf. So only buffer the output if the output is piped to the next process.
---
 src/builtin_eval.cpp | 61 +++++++++++++++++++++++++++++---------------
 1 file changed, 40 insertions(+), 21 deletions(-)

diff --git a/src/builtin_eval.cpp b/src/builtin_eval.cpp
index 4beda0548..ead61dce4 100644
--- a/src/builtin_eval.cpp
+++ b/src/builtin_eval.cpp
@@ -27,24 +27,40 @@ int builtin_eval(parser_t &parser, io_streams_t &streams, wchar_t **argv) {
         new_cmd += argv[i];
     }
 
-    // The output and errput of eval must go to our streams, not to the io_chain in our streams,
-    // because they may contain a pipe which is intended to be consumed by a process which is not
-    // yet launched (#6806). Make bufferfills to capture it.
-    // TODO: we are sloppy and do not honor other redirections; eval'd code won't see for example a
-    // redirection of fd 3.
-    shared_ptr<io_bufferfill_t> stdout_fill =
-        io_bufferfill_t::create(fd_set_t{}, parser.libdata().read_limit, STDOUT_FILENO);
-    shared_ptr<io_bufferfill_t> stderr_fill =
-        io_bufferfill_t::create(fd_set_t{}, parser.libdata().read_limit, STDERR_FILENO);
-    if (!stdout_fill || !stderr_fill) {
-        // This can happen if we are unable to create a pipe.
-        return STATUS_CMD_ERROR;
+    // If stdout is piped, then its output must go to the streams, not to the io_chain in our
+    // streams, because the pipe may be intended to be consumed by a process which
+    // is not yet launched (#6806). If stdout is NOT redirected, it must see the tty (#6955). So
+    // create a bufferfill for stdout if and only if stdout is piped.
+    // Note do not do this if stdout is merely redirected (say, to a file); we don't want to
+    // buffer in that case.
+    shared_ptr<io_bufferfill_t> stdout_fill{};
+    if (streams.out_is_piped) {
+        stdout_fill =
+            io_bufferfill_t::create(fd_set_t{}, parser.libdata().read_limit, STDOUT_FILENO);
+        if (!stdout_fill) {
+            // We were unable to create a pipe, probably fd exhaustion.
+            return STATUS_CMD_ERROR;
+        }
     }
 
+    // Of course the same applies to stderr.
+    shared_ptr<io_bufferfill_t> stderr_fill{};
+    if (streams.err_is_piped) {
+        stderr_fill =
+            io_bufferfill_t::create(fd_set_t{}, parser.libdata().read_limit, STDERR_FILENO);
+        if (!stderr_fill) {
+            return STATUS_CMD_ERROR;
+        }
+    }
+
+    // Construct the full io chain, perhaps with our bufferfills appended.
+    io_chain_t ios = *streams.io_chain;
+    if (stdout_fill) ios.push_back(stdout_fill);
+    if (stderr_fill) ios.push_back(stderr_fill);
+
     const auto cached_exec_count = parser.libdata().exec_count;
     int status = STATUS_CMD_OK;
-    if (parser.eval(new_cmd, io_chain_t{stdout_fill, stderr_fill}, block_type_t::top,
-                    streams.parent_pgid) != eval_result_t::ok) {
+    if (parser.eval(new_cmd, ios, block_type_t::top, streams.parent_pgid) != eval_result_t::ok) {
         status = STATUS_CMD_ERROR;
     } else if (cached_exec_count == parser.libdata().exec_count) {
         // Issue #5692, in particular, to catch `eval ""`, `eval "begin; end;"`, etc.
@@ -55,14 +71,17 @@ int builtin_eval(parser_t &parser, io_streams_t &streams, wchar_t **argv) {
     }
 
     // Finish the bufferfills - exhaust and close our pipes.
+    // Copy the output from the bufferfill back to the streams.
     // Note it is important that we hold no other references to the bufferfills here - they need to
     // deallocate to close.
-    std::shared_ptr<io_buffer_t> output = io_bufferfill_t::finish(std::move(stdout_fill));
-    std::shared_ptr<io_buffer_t> errput = io_bufferfill_t::finish(std::move(stderr_fill));
-
-    // Copy the output from the bufferfill back to the streams.
-    streams.out.append_narrow_buffer(output->buffer());
-    streams.err.append_narrow_buffer(errput->buffer());
-
+    ios.clear();
+    if (stdout_fill) {
+        std::shared_ptr<io_buffer_t> output = io_bufferfill_t::finish(std::move(stdout_fill));
+        streams.out.append_narrow_buffer(output->buffer());
+    }
+    if (stderr_fill) {
+        std::shared_ptr<io_buffer_t> errput = io_bufferfill_t::finish(std::move(stderr_fill));
+        streams.err.append_narrow_buffer(errput->buffer());
+    }
     return status;
 }